[Mono.Android] Bind Build.getVersion() as Build.GetVersion()

[jonpryor]

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=60069
Context: https://bugzilla.xamarin.com/attachment.cgi?id=25276

API-26 added an android.os.Build.getSerial() method. Under
normal "name mangling" conventions, generator binds this as an
Android.OS.Build.Serial property.

There's just one "minor" problem with this: there is also an
android.os.Build.SERIAL field, since API-9, which generator
also binds as an Android.OS.Build.Serial property.

Thus, a question: given the Java code:

public class Build {
	public static String SERIAL = "UNKNOWN";
	public static String getSerial() {/* ... */ }
}

What will generator do?

Answer: the wrong thing: the field is ignored, and the
Build.getSerial() method is used for the Build.Serial property.

This is problematic. The managed API has not changed -- pre-API-26
and post-API-26 both have a string Build.Serial property --
but the behavior of the method has changed from reading the value of
a static field, to invoking a static method.

A static method that only exists on devices running API-26 and later.

Meaning:

1. Create a new Xamarin.Android app.
2. Set $(TargetFrameworkVersion) to v8.0, and
   //uses-sdk/@android:minSdkVersion to 11 within
   AndroidManifest.xml.
3. Within e.g. MainActivity.OnCreate(), reference the
   Android.OS.Build.Serial property.
4. Build the app, and run on e.g. an Android 7.1 device.

Expected result: it works!

Actual result: not so much:

Java.Lang.NoSuchMethodError: no static method "Landroid/os/Build;.getSerial()Ljava/lang/String;"

@atsushieno has determined that this is the only such behavioral
breakage introduced by API-26. His
referenced metadata fixup changes patch lists all the fields
which are currently being ignored by the Mono.Android build process.

In the interest of expediency (and getting this merged into d15-4),
include only the metadata changes that impact Build.getSerial(),
and force it to be bound as a Build.GetSerial() method. This
"un-hides" the Build.Serial binding of the Build.SERIAL field,
restoring semantic compatibility.

The remaining of the metadata fixup changes should be merged
separately.