[Mono.Android] remove S.R.Emit usage in TypeManager.Activate()

[jonathanpeppers]

Context: https://github.com/microsoft/dotnet-podcasts

Reviewing a dotnet trace log from the .NET Podcast app, I see:

40.12ms Mono.Android!Java.Interop.TypeManager.n_Activate(intptr,intptr,intptr,intptr,intptr,intptr)

The bulk of this time (~33ms) is spent in System.Reflection.Emit-related code.

Instead of using S.R.E, it appears we can use regular
System.Reflection and call ConstructorInfo.Invoke() to do the work?

The equivalent, non S.R.E code would call:

• RuntimeHelpers.GetUninitializedObject()
• Set the field .handle directly, which requires
  Java.Lang.Object.handle to be internal.
• Invoke the ConstructorInfo, passing the uninitialized object

After these changes, a new dotnet trace of the .NET Podcast app:

17.09ms Mono.Android!Java.Interop.TypeManager.n_Activate(intptr,intptr,intptr,intptr,intptr,intptr)

I see about 6.37ms of the time spent in regular System.Reflection code
instead (ConstructorInfo.Invoke()).

Results

Testing dotnet new maui on a Pixel 5 device, a regular Release
build (without AOT):

Before:
02-16 11:32:15.665  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +970ms
02-16 11:32:16.797  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +940ms
02-16 11:32:17.993  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +943ms
02-16 11:32:19.191  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +949ms
02-16 11:32:20.436  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +963ms
02-16 11:32:21.600  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +940ms
02-16 11:32:22.764  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +934ms
02-16 11:32:23.962  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +959ms
02-16 11:32:25.097  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +942ms
02-16 11:32:26.290  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +941ms
Average(ms): 948.1
Std Err(ms): 3.74892577206384
Std Dev(ms): 11.855144218627

After:
02-16 11:23:01.038  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +918ms
02-16 11:23:02.250  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +929ms
02-16 11:23:03.432  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +926ms
02-16 11:23:04.614  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +942ms
02-16 11:23:05.796  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +937ms
02-16 11:23:07.005  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +948ms
02-16 11:23:08.211  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +955ms
02-16 11:23:09.353  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +932ms
02-16 11:23:10.555  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +954ms
02-16 11:23:11.729  1980  2259 I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +946ms
Average(ms): 938.7
Std Err(ms): 3.91875830674292
Std Dev(ms): 12.3922018490124

I suspect this saves ~10ms for this app?

[jonpryor]

Context: https://github.com/microsoft/dotnet-podcasts
Context: https://github.com/xamarin/monodroid/commit/0c29e9a1eb6572082dd478f34bad092049974a00

Reviewing a `dotnet trace` log from the .NET Podcast app, I see:

	40.12ms Mono.Android!Java.Interop.TypeManager.n_Activate(intptr,intptr,intptr,intptr,intptr,intptr)

The bulk of this time (~33ms) is spent in `System.Reflection.Emit`-
related code.

Instead of using `System.Reflection.Emit`, it appears we can instead
use regular `System.Reflection` and instead call
[`MethodBase.Invoke(object?, object?[]?)`][0] on a `ConstructorInfo`
instance to invoke a constructor on an already created instance.

The equivalent, non-`System.Reflection.Emit` code would call:

 1. `RuntimeHelpers.GetUninitializedObject()`

 2. Set the field `.handle` directly, which requires
    `Java.Lang.Object.handle` and `Java.Lang.Throwable.handle` to be
    `internal`.

 3. Invoke "`ConstructorInfo.Invoke()`", *but actually* the
    `MethodBase.Invoke(object?, object?[]?)` overload, providing the
    uninitialzied object as the first `obj` parameter.

After these changes, a new `dotnet trace` of the .NET Podcast app:

	17.09ms Mono.Android!Java.Interop.TypeManager.n_Activate(intptr,intptr,intptr,intptr,intptr,intptr)

I see about 6.37ms of the time spent in regular `System.Reflection`
code (`ConstructorInfo.Invoke()`) instead of `DynamicMethod` code.

~~ Results ~~

Testing `dotnet new maui` on a Pixel 5 device, a regular `Release`
build (without AOT):

  * Before:

        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +970ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +940ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +943ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +949ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +963ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +940ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +934ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +959ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +942ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +941ms

      * Average(ms): 948.1
      * Std Err(ms): 3.74892577206384
      * Std Dev(ms): 11.855144218627

  * After:

        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +918ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +929ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +926ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +942ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +937ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +948ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +955ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +932ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +954ms
        I ActivityTaskManager: Displayed com.companyname.foo/crc64808a40cc7e533249.MainActivity: +946ms

      * Average(ms): 938.7
      * Std Err(ms): 3.91875830674292
      * Std Dev(ms): 12.3922018490124

I suspect this saves ~10ms for this app?

~~ Meta Question ~~

This change necessitates asking why `DynamicMethod` was used in
the first place.  The [commit which introduced the change][1] states:

> Implement a DynamicMethod based constructor trampoline for Java 
> based activation
> 
> We need to guard against reentrancy of java-activated objects, so
> we build a small dynamic method which creates the uninitialized
> object, sets the handle field and then runs the constructor.
> This allows us to guard against calling NewObject based on the
> value of the IntPtr field.

[`JNIEnv::NewObject()`][2] is Bad™; [`JNIEnv::AllocObject()`][3] and
[`JNIEnv::CallNonvirtualVoidMethod()`][4] should be used instead.
See [xamarin/java.interop@972c5bc6][5] for details.

However, Xamarin.Android's use of `JNIEnv::AllocObject()` +
`JNIEnv::CallNonvirtualVoidMethod()` wasn't introduced [until 2013][6],
while the initial use of `DynamicMethod` began in 2010.

Thus, two theories for why `DynamicMethod` was used:

 1. We didn't realize that `ConstructorInfo` inherited from
    `MethodBase`, that `MethodBase.Invoke(object, object[])` existed,
    and thus we avoid `DynamicMethod` entirely, *or*

 2. `DynamicMethod` was an attempt to work around the limitations of
    `JNIEnv::NewObject()`, before all the pitfalls of using
    `JNIEnv::NewObject()` were fully understood.  (Additionally,
    `JNIEnv::NewObject()` use was *required* before Android 3.0!)

Either way, it doesn't look like we need to use `DynamicMethod` to
invoke C# constructors anymore.

[0]: https://docs.microsoft.com/en-us/dotnet/api/system.reflection.methodbase.invoke?view=net-6.0#system-reflection-methodbase-invoke(system-object-system-object())
[1]: https://github.com/xamarin/monodroid/commit/0c29e9a1eb6572082dd478f34bad092049974a00
[2]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewObject
[3]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#AllocObject
[4]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#CallNonvirtual_type_Method_routines
[5]: https://github.com/xamarin/Java.Interop/commit/972c5bc605bd87526f3ddd7735d21f47d9f2d4aa
[6]: https://github.com/xamarin/monodroid/commit/326509e56d4e582c53bbe5dfe6d5c741a27f1af5