[Java.Interop] Emit a reference assembly for Java.Interop.dll (#716)

Context: dotnet/android@00c2c1a

The `Microsoft.Android.Ref` package for .NET 6 needs to contain
reference assemblies for each assembly we ship.

`Java.Interop.dll` is one of the assemblies included in the
`Microsoft.Android.Ref` package, so we need to set
`$(ProduceReferenceAssembly)`=True so that a reference assembly is
created automatically.

Size difference:

	Length Name
	------ ----
	227328 Java.Interop.dll
	 94208 ref/Java.Interop.dll

Author: jonathanpeppers

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

@@ -15,6 +15,7 @@
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
     <LangVersion>8.0</LangVersion>
+    <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
   </PropertyGroup>
   <Import Project="..\..\Directory.Build.props" />
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">

src/Java.Interop/Java.Interop.csproj

@@ -14,6 +14,7 @@
     <JNIEnvGenPath>$(BuildToolOutputFullPath)</JNIEnvGenPath>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
+    <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <DefineConstants>DEBUG;$(DefineConstants)</DefineConstants>