[Java.Interop-MonoAndroid.csproj] Add jnienv-gen ProjectReference (#990)

Context: dotnet/android#7068

When attempting to build xamarin/xamarin-android *without* using
`dotnet build -m:1` -- *with* parallel builds enabled -- the build
can randomly fail:

	'"C:\a\_work\1\s\external\Java.Interop\bin\BuildRelease\jnienv-gen.exe"' is not recognized as an internal or external command, operable program or batch file.

or

	…/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop.targets(19,5):
	error MSB3073: The command "mono "…/xamarin-android/external/Java.Interop/bin/BuildDebug/jnienv-gen.exe" Java.Interop/JniEnvironment.g.cs obj/Debug/jni.c" exited with code 2.
	[…/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop-MonoAndroid.csproj]

The cause of the error is that xamarin-android's `Mono.Android.targets`
[uses `<MSBuild/>` to build `Java.Interop-MonoAndroid.csproj`][0],
and `Java.Interop-MonoAndroid.csproj` does not have a
`@(ProjectReference)` to `build-tools/jnienv-gen/jnienv-gen.csproj`.
Consequently, when `Java.Interop-MonoAndroid.csproj` is built,
`jnienv-gen.exe` *may not exist*, resulting in the MSB3073 errors.

Fix this usage scenario by updating `Java.Interop-MonoAndroid.csproj`
to have a `@(ProjectReference)` to
`build-tools/jnienv-gen/jnienv-gen.csproj`.  This ensures that
`jnienv-gen.exe` exists before the `BuildJniEnvironment_g_cs` target
in `Java.Interop.targets` is executed.

Additionally, move some of the "property overrides" defined in
`Mono.Android.targets` (for use by `Java.Interop-MonoAndroid.csproj`)
into `Java.Interop-MonoAndroid.csproj`.  The problem with having them
in `Mono.Android.targets` is that the invocation worked as a *global*
override, overriding `$(TargetFrameworkVersion)`/etc. when
*`build-tools/jnienv-gen/jnienv-gen.csproj`* is built, causing the
resulting `jnienv-gen.exe` to possibly be a "monoandroid10" app,
*not* a net472 app.  Moving these properties into
`Java.Interop-MonoAndroid.csproj` means that `Mono.Android.targets`
*doesn't* need to specify them, which means the `jnienv-gen.csproj`
build won't inadvertently use them.

[0]: https://github.com/xamarin/xamarin-android/blob/6b43af37bc57b7eea2a213ee15de037e61784a1c/src/Mono.Android/Mono.Android.targets#L23-L50

Author: jpobst

src/Java.Interop/Java.Interop-MonoAndroid.csproj

@@ -1,5 +1,12 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <PropertyGroup Condition=" '$(XAInstallPrefix)' != '' ">
+    <TargetFrameworkIdentifier>MonoAndroid</TargetFrameworkIdentifier>
+    <TargetFrameworkVersion>v1.0</TargetFrameworkVersion>
+    <TargetFrameworkRootPath>$(XAInstallPrefix)xbuild-frameworks</TargetFrameworkRootPath>
+  </PropertyGroup>
+  
   <PropertyGroup>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
     <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
@@ -113,5 +120,10 @@
     <None Include="Documentation\Java.Interop\IJavaPeerable.xml" />
     <None Include="Documentation\Java.Interop\JniManagedPeerStates.xml" />
     <None Include="Documentation\Java.Interop\JniEnvironment.References.xml" />
+    <ProjectReference Include="..\..\build-tools\jnienv-gen\jnienv-gen.csproj"
+        ReferenceOutputAssembly="false"
+        SkipGetTargetFrameworkProperties="True"
+        AdditionalProperties="TargetFramework=net472"
+    />
   </ItemGroup>
 </Project>