OpenJDK11 support!

It builds locally!  (If that actually means anything.)

(Unit tests untested.)

Make "global" `$(JavacSourceVersion)` and `$(JavacTargetVersion)`
MSBuild properties for great consistency!

Use a new `$(JavacTargetVersion)` value for `javac -target` instead of
reusing `$(JavacSourceVersion)`, because...why not?

Cleanup `build-tools/scripts/Jar.targets` to *not* change the behavior
depending on what the installed JDK is.  JDK 1.8 is now a *minimum* to
*build* xamarin-android...and probably to *run* any Java code that we
build.  (Maybe.)

Various fixes to allow dotnet/java-interop@6d7266dd9 to build.

Configuration.props

@@ -118,6 +118,10 @@
     <MingwCommandPrefix32>i686-w64-mingw32</MingwCommandPrefix32>
     <MingwCommandPrefix64>x86_64-w64-mingw32</MingwCommandPrefix64>
   </PropertyGroup>
+  <PropertyGroup>
+    <JavacSourceVersion>1.8</JavacSourceVersion>
+    <JavacTargetVersion>1.8</JavacTargetVersion>
+  </PropertyGroup>
   <PropertyGroup>
     <AndroidMxeFullPath Condition=" '$(AndroidMxeInstallPrefix)' != '' ">$([System.IO.Path]::GetFullPath ('$(AndroidMxeInstallPrefix)'))</AndroidMxeFullPath>
     <AndroidNdkFullPath>$([System.IO.Path]::GetFullPath ('$(AndroidNdkDirectory)'))</AndroidNdkFullPath>

build-tools/scripts/Jar.targets

@@ -3,10 +3,6 @@
   <Target Name="_GetJavacVersions"
       DependsOnTargets="AndroidPrepareForBuild">
     <PropertyGroup>
-      <_JavacSourceVersion Condition="$(_JdkVersion.StartsWith ('9'))">1.8</_JavacSourceVersion>
-      <_JavacSourceVersion Condition=" '$(_JavacSourceVersion)' == '' ">1.5</_JavacSourceVersion>
-      <_JavacTargetVersion Condition="$(_JdkVersion.StartsWith ('9'))">1.8</_JavacTargetVersion>
-      <_JavacTargetVersion Condition=" '$(_JavacTargetVersion)' == '' ">1.6</_JavacTargetVersion>
       <JarPath Condition=" '$(JarPath)' == '' ">$(JavaSdkDirectory)\bin\jar</JarPath>
       <JavaCPath Condition=" '$(JavaCPath)' == '' ">$(JavaSdkDirectory)\bin\javac</JavaCPath>
     </PropertyGroup>
@@ -35,7 +31,7 @@
     <PropertyGroup>
       <_Javac>"$(JavaCPath)"</_Javac>
       <_Jar>"$(JarPath)"</_Jar>
-      <_Targets>-source $(_JavacSourceVersion) -target $(_JavacTargetVersion)</_Targets>
+      <_Targets>-source $(JavacSourceVersion) -target $(JavacTargetVersion)</_Targets>
       <_DestDir>$(IntermediateOutputPath)__CreateTestJarFile-bin</_DestDir>
       <_AndroidJar>-bootclasspath "$(AndroidSdkDirectory)\platforms\android-$(_AndroidApiLevelName)\android.jar"</_AndroidJar>
       <_CP>-cp "$(_JavaInteropJarPath)"</_CP>

build-tools/scripts/JavaCallableWrappers.targets

@@ -31,7 +31,7 @@
         Overwrite="True"
     />
     <PropertyGroup>
-      <_Target>-source $(JavacSourceVersion) -target $(JavacSourceVersion)</_Target>
+      <_Target>-source $(JavacSourceVersion) -target $(JavacTargetVersion)</_Target>
       <_D>-d "$(IntermediateOutputPath)jcw\bin"</_D>
       <_AndroidJar>"$(AndroidToolchainDirectory)\sdk\platforms\android-$(AndroidPlatformId)\android.jar"</_AndroidJar>
       <_MonoAndroidJar>$(OutputPath)mono.android.jar</_MonoAndroidJar>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.props.in

@@ -7,7 +7,7 @@
 		<ImplicitlyExpandNETStandardFacades>false</ImplicitlyExpandNETStandardFacades>
 		<CopyNuGetImplementations Condition=" '$(CopyNuGetImplementations)' == ''">true</CopyNuGetImplementations>
 		<YieldDuringToolExecution Condition="'$(YieldDuringToolExecution)' == ''">true</YieldDuringToolExecution>
-		<LatestSupportedJavaVersion Condition="'$(LatestSupportedJavaVersion)' == ''">1.8.0</LatestSupportedJavaVersion>
+		<LatestSupportedJavaVersion Condition="'$(LatestSupportedJavaVersion)' == ''">11.99.99</LatestSupportedJavaVersion>
 		<MinimumSupportedJavaVersion Condition="'$(MinimumSupportedJavaVersion)' == ''">1.6.0</MinimumSupportedJavaVersion>
 		<AndroidVersionCodePattern Condition=" '$(AndroidUseLegacyVersionCode)' != 'True' And '$(AndroidVersionCodePattern)' == '' ">{abi}{versionCode:D5}</AndroidVersionCodePattern>
 		<AndroidResourceGeneratorTargetName>UpdateGeneratedFiles</AndroidResourceGeneratorTargetName>

src/java-runtime/java-runtime.targets

@@ -33,7 +33,7 @@
       Overwrite="True"
   />
   <PropertyGroup>
-    <_Target Condition="'$(JavacSourceVersion)' != ''">-source $(JavacSourceVersion) -target $(JavacSourceVersion)</_Target>
+    <_Target Condition="'$(JavacSourceVersion)' != ''">-source $(JavacSourceVersion) -target $(JavacTargetVersion)</_Target>
     <_AndroidJar>"$(AndroidToolchainDirectory)\sdk\platforms\android-$(AndroidFirstPlatformId)\android.jar"</_AndroidJar>
   </PropertyGroup>
   <Exec