[build] JavacSourceVersion JavacTargetVersion Harmonization

[jonpryor]

Context: #4567

In order to support using JDK 11 to build Xamarin.Android, we need to
consistently use javac -source 1.8 -target 1.8 ~everywhere, because
we also require javac --boot-class-path, and JDK11 doesn't support
javac --boot-class-path unless targeting < JDK 1.9:

% javac --boot-class-path ~/android-toolchain/sdk/platforms/android-29/android.jar Example.java
error: option --boot-class-path not allowed with target 11

The <Javac/> task use javac --boot-class-path to compile Java
code, so that Android's android.jar provides everything:

Task "Javac" (TaskId:154)
  Task Parameter:JavaPlatformJarPath=/Users/runner/Library/Android/sdk/platforms/android-29/android.jar
  Task Parameter:ClassesOutputDirectory=obj/Release/android/bin/classes/
  Task Parameter:ClassesZip=obj/Release/android/bin/classes.zip
  Task Parameter:StubSourceDirectory=obj/Release/android/src/
  Task Parameter:ToolPath=/Users/runner/Library/Android/jdk/bin
  Task Parameter:
      Jars=
          /Library/Frameworks/Mono.framework/External/xbuild-frameworks/MonoAndroid/v10.0/mono.android.jar
          /Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/java_runtime.jar
  /Users/runner/Library/Android/jdk/bin/javac -J-Dfile.encoding=UTF8 "@/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmp2eba46d8.tmp"
JAVAC : error : option --boot-class-path not allowed with target 11
  The command exited with code 2. (TaskId:154)

Harmonize provisioning of javac -source and javac -target values:

• Add $(JavacSourceVersion) and $(JavacTargetVersion) MSBuild
  properties to Configuration.props.

• Update the src/manifestmerger and src/r8 builds so that
  $(JavacSourceVersion) and $(JavacTargetVersion) are used to
  control the .class files that they create.

• Update build-tools/scripts/Jar.targets to use the values from
  Configuration.props, instead of separate values.

• Fix errant invocations of javac -target $(JavacSourceVersion),
  which should be javac -target $(JavacTargetVersion).