JetBrains OpenJDK 11

[grendello]

This PR is a test to see what works and what breaks with JetBrains
OpenJDK 11 as far as Xamarin.Android is concerned.

[jonpryor]

The build fails, perhaps unsurprisingly?

  "/Users/builder/Library/Android/jdk/bin/javac" -source 1.6 -target 1.6 -bootclasspath "/Users/builder/Library/Android/jdk/bin/../jre/lib/rt.jar" -d "obj/Release/netcoreapp3.1/ji-classes" java/com/xamarin/java_interop/internal/JavaProxyObject.java java/com/xamarin/java_interop/internal/JavaProxyThrowable.java java/com/xamarin/java_interop/GCUserPeerable.java java/com/xamarin/java_interop/ManagedPeer.java
EXEC : warning : [options] source value 6 is obsolete and will be removed in a future release [/Users/builder/azdo/_work/3/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop.csproj]
EXEC : warning : [options] target value 1.6 is obsolete and will be removed in a future release [/Users/builder/azdo/_work/3/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop.csproj]
EXEC : warning : [options] To suppress warnings about obsolete options, use -Xlint:-options. [/Users/builder/azdo/_work/3/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop.csproj]
  3 warnings
EXEC : Fatal error : Unable to find package java.lang in classpath or bootclasspath [/Users/builder/azdo/_work/3/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop.csproj]

This tries to use /Users/builder/Library/Android/jdk/bin/../jre/lib/rt.jar as the -bootclasspath value, and this path doesn't exist…which doesn't seem that surprising, given dotnet/android-tools@36d7fee, which "replaces" jre with lib.

Though that isn't quite "enough"; https://bintray.com/jetbrains/intellij-jdk/download_file?file_path=jbrsdk-11_0_4-osx-x64-b546.1.tar.gz doesn't have an rt.jar file at all!

A quick grep -ril String.class ., and the only seemingly relevant file is lib/modules.

Regardless, every use of javac -bootclasspath across both Java.Interop and xamarin-android needs to be audited for correctness. A quick glance suggests that all xamarin-android uses use android.jar, which presumably is still fine, but Java.Interop expects $(JreRtJarPath) to be property set: https://github.com/xamarin/java.interop/blob/c0948d2a19eba5f50e53888ef070a97ac504f06a/Directory.Build.props#L49

and $(JreRtJarPath) hardcodes a path which doesn't exist. :-(

See also: https://stackoverflow.com/questions/46438557/how-to-extract-the-file-jre-9-lib-modules

[grendello]

It seems -bootclasspath is forbidden:

"/Users/builder/Library/Android/jdk/bin/javac"  -d obj/Release/release -bootclasspath "/Users/builder/Library/Android/sdk/platforms/android-19/android.jar":"/Users/builder/azdo/_work/3/s/xamarin-android/bin/Release/lib/xamarin.android/xbuild/Xamarin/Android/java_runtime.jar" @obj/Release/release.txt
EXEC : error : option --boot-class-path not allowed with target 11 [/Users/builder/azdo/_work/3/s/xamarin-android/src/java-runtime/java-runtime.csproj]

[grendello]

Moved to: #4567