[java-runtime] Use r8.jar instead of dx

`dx` doesn't like OpenJDK 11 on Windows:

	  "C:\Users\dlab14\android-toolchain\sdk\build-tools\29.0.2\dx" --dex --no-strict --output="C:\A\vs2019xam00000H-1\_work\2\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\java_runtime.dex" "C:\A\vs2019xam00000H-1\_work\2\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\java_runtime.jar"

	##[debug]Processed: ##vso[task.logdetail id=dffb25d4-cb0b-4e58-94d2-4b9a4f215bed;parentid=;name=EXEC;type=Build;starttime=2020-04-19T01:21:10.0332656Z;state=InProgress;]
	##[error]EXEC(0,0): Error : No suitable Java found. In order to properly use the Android Developer
	##[debug]Processed: ##vso[task.logissue type=Error;sourcepath=EXEC;linenumber=0;columnnumber=0;code=;]No suitable Java found. In order to properly use the Android Developer
	##[debug]Processed: ##vso[task.logdetail id=dffb25d4-cb0b-4e58-94d2-4b9a4f215bed;parentid=;type=Build;result=Failed;finishtime=2020-04-19T01:21:10.0488457Z;progress=100;state=Completed;parentid=;name=;]
	EXEC : error : No suitable Java found. In order to properly use the Android Developer [C:\A\vs2019xam00000H-1\_work\2\s\src\java-runtime\java-runtime.csproj]
	  Tools, you need a suitable version of Java JDK installed on your system.
	  We recommend that you install the JDK version of JavaSE, available here:
	    http://www.oracle.com/technetwork/java/javase/downloads

	  If you already have Java installed, you can define the JAVA_HOME environment
	  variable in Control Panel / System / Avanced System Settings to point to the
	  JDK folder.

	  You can find the complete Android SDK requirements here:
	    http://developer.android.com/sdk/requirements.html

	##[error]src\java-runtime\java-runtime.targets(59,3): Error MSB3073: The command ""C:\Users\dlab14\android-toolchain\sdk\build-tools\29.0.2\dx" --dex --no-strict --output="C:\A\vs2019xam00000H-1\_work\2\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\java_runtime.dex" "C:\A\vs2019xam00000H-1\_work\2\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\java_runtime.jar"" exited with code -1.

Use `r8.jar` instead, to hopefully avoid the problem.

Author: jonpryor

src/java-runtime/java-runtime.targets

@@ -57,8 +57,12 @@
     Inputs="@(_RuntimeOutput->'%(Identity)')"
     Outputs="@(_RuntimeOutput->'%(OutputDex)')">
   <Exec
-      Command="&quot;$(AndroidSdkDirectory)\build-tools\$(XABuildToolsFolder)\dx&quot; --dex --no-strict --output=&quot;%(_RuntimeOutput.OutputDex)&quot; &quot;%(_RuntimeOutput.OutputJar)&quot;"
+      Command="&quot;$(JavaPath)&quot; -classpath &quot;$(OutputPath)\r8.jar&quot; com.android.tools.r8.D8 --release --no-desugaring --output &quot;%(_RuntimeOutput.IntermediateRuntimeOutputPath)&quot; &quot;%(_RuntimeOutput.OutputJar)&quot;"
       EnvironmentVariables="JAVA_HOME=$(JavaSdkDirectory)"
   />
+  <Move
+      SourceFiles="@(_RuntimeOutput->'%(IntermediateRuntimeOutputPath)\classes.dex')"
+      DestinationFiles="%(_RuntimeOutput.OutputDex)"
+  />
 </Target>
 </Project>