[build] Support building with JetBrains OpenJDK 11

[jonpryor]

Context: https://issuetracker.google.com/issues/150189789
Context: dotnet/android#4562
Context: dotnet/android#4567

Bumps to xamarin/xamarin-android-tools/master@36d7fee5

Changes: dotnet/android-tools@bfb66f3...36d7fee

• dotnet/android-tools@36d7fee: JetBrains OpenJDK 11 detection (Cosmetic xml adjuster fixes #82)
• dotnet/android-tools@12f52ac: Merge pull request [generator] generate only generatable code. (#73) #80 from jonpryor/jonp-drop-net461
• dotnet/android-tools@c7090d0: [Xamarin.Android.Tools.AndroidSdk] Remove net461

JDK 9 -- released 2017-July-27 -- introduced many new features, but
broke various Android SDK toolchain programs in various inscrutable
ways, so the Android community has been "stuck" on JDK 8 ever since.

…until now? A preview version of apksigner in the Build-tools 30rc1
package states that it requires Java 9 in order to run, which means we
must explore what is required to build under JDK > 8.

JetBrains has an OpenJDK 11.0.4 release for macOS, which has a
"weird" directory structure but is otherwise workable, so…

Will It Build™?

$ curl -o jbrsdk-11_0_4-osx-x64-b546.1.tar.gz https://bintray.com/jetbrains/intellij-jdk/download_file?file_path=jbrsdk-11_0_4-osx-x64-b546.1.tar.gz
# Above doesn't *actually* work; use a browser to appease Akamai
$ tar xzf jbrsdk-11_0_4-osx-x64-b546.1.tar.gz
$ export JAVA_HOME=`pwd`/jbrsdk/Contents/Home
$ make prepare JI_MAX_JDK=12
$ make all

Yes, it builds, but that's misleading: it's not actually using
$JAVA_HOME!

…/Java.Interop/build-tools/scripts/jdk.targets(5,5): warning : Not a valid JDK directory: `…/Java.Interop/jbrsdk/Contents/Home`; via locator: $JAVA_HOME
System.ArgumentException: Could not find required file `jvm` within `…/Java.Interop/jbrsdk/Contents/Home`; is this a valid JDK?
Parameter name: homePath
  at Xamarin.Android.Tools.JdkInfo.ValidateFile (System.String name, System.String path)
  at Xamarin.Android.Tools.JdkInfo..ctor (System.String homePath)

This is fixed via dotnet/android-tools@36d7fee. Bump
xamarin-android-tools, and make prepare still works.

make all fails:

$ make all
…
  "…/Java.Interop/jbrsdk/Contents/Home/bin/javac" -parameters -source 1.6 -target 1.6 -bootclasspath "…/Java.Interop/jbrsdk/Contents/Home/bin/../jre/lib/rt.jar" -g -d "obj/Debug/classes" java/android/annotation/NonNull.java java/android/annotation/NonNull.java java/com/xamarin/IJavaInterface.java java/com/xamarin/IParameterInterface.java java/com/xamarin/JavaAnnotation.java java/com/xamarin/JavaType.java java/com/xamarin/NestedInterface.java java/com/xamarin/NotNullClass.java java/com/xamarin/ParameterAbstractClass.java java/com/xamarin/ParameterClass.java java/com/xamarin/ParameterClass2.java java/java/util/Collection.java java/NonGenericGlobalType.java
EXEC : warning : [options] source value 6 is obsolete and will be removed in a future release
EXEC : warning : [options] target value 1.6 is obsolete and will be removed in a future release
EXEC : warning : -parameters is not supported for target value 1.6. Use 1.8 or later.
EXEC : warning : [options] To suppress warnings about obsolete options, use -Xlint:-options.
  4 warnings
EXEC : Fatal error : Unable to find package java.lang in classpath or bootclasspath

The problem is that JetBrains' OpenJDK 11 no longer contains a
…/jre/lib/rt.jar file, so the -bootclasspath value is now wrong.

Additionally, if you don't use javac -target, which implicitly
targets JDK 11, javac doesn't like that:

EXEC : error : option --boot-class-path not allowed with target 11

The solution? Don't Do That™; if javac from OpenJDK 11 doesn't want
-bootclasspath, don't provide it.

Update the <JdkInfo/> task to check for the existence of the
…/jre/lib/rt.jar file; if it exists, set $(JreRtJarPath),
otherwise the $(JreRtJarPath) MSBuild property is empty.

Then update $(_JavacSourceOptions) so that it doesn't provide
-bootclasspath when $(JreRtJarPath) is empty.

These three changes -- xamarin-android-tools bump, <JdkInfo/>
update, and $(_JavacSourceOptions) update -- allow make all to
build successfully.

Then we hit unit tests. The above javac invocation has a warning:

EXEC : warning : -parameters is not supported for target value 1.6. Use 1.8 or later.

Even though we've been using javac -target 1.6 -parameters for
ages, this combination is no longer supported. In order to use
javac -parameters now, we need to use javac -target 1.8.

Update $(JavacSourceVersion) to 1.8 so that we can continue using
javac -parameters, which in turn requires that we update
tests/Xamarin.Android.Tools.Bytecode-Tests so that we expect the
newly updated .class file MajorVersion values.