[Mono.Android] Build latest stable version against netcoreapp3.1 (#4419)

We expect the .NET 5 BCL to closely resemble the .NET Core 3.1 BCL.
Continue preparing for .NET 5 and build `Mono.Android.dll` against
the `netcoreapp3.1` target framework, to ensure that the APIs that we
need actually exist.

Additionally, the `Mono.Android.csproj` build will effectively be a
no-op when targeting `netcoreapp3.1` if `$(AndroidFrameworkVersion)`
is not equal to `$(AndroidLatestStableFrameworkVersion)`, as we
currently plan on only shipping *one* `Mono.Android.dll` assembly as
part of .NET 5, so there is no need to build `Mono.Android.dll`
against other `$(TargetFrameworkVersion)` values.

When building for `netcoreapp3.1`, `Mono.Android.dll` uses the
`Java.Interop.dll` which is built for .NET Standard, instead of the
one built against the `MonoAndroid` framework.  We may want to
consider updating `Java.Interop.csproj` to multitarget against
`netcoreapp3.1` as well.

Author: pjcollins

build-tools/scripts/JavaCallableWrappers.targets

@@ -5,11 +5,17 @@
       Inputs="$(JavaCallableWrapperAbsAssembly);@(JavaCallableWrapperSource)"
       Outputs="$(OutputPath)mono.android.jar">
     <MakeDir Directories="$(IntermediateOutputPath)jcw;$(IntermediateOutputPath)jcw\bin" />
+    <ItemGroup>
+      <_CorlibPath Include="@(ReferencePath->'%(RootDir)%(Directory)')" Condition=" '%(FileName)%(Extension)' == 'mscorlib.dll' " />
+    </ItemGroup>
     <PropertyGroup>
+      <_CorlibDir>@(_CorlibPath)</_CorlibDir>
       <OutputPathAbs Condition="$([System.IO.Path]::IsPathRooted($(OutputPath)))">$(OutputPath)</OutputPathAbs>
       <OutputPathAbs Condition=" '$(OutputPathAbs)' == '' ">$(MSBuildProjectDirectory)\$(OutputPath)</OutputPathAbs>
       <JcwGen>"$(XAInstallPrefix)xbuild\Xamarin\Android\jcw-gen.exe" -v10</JcwGen>
-      <_LibDirs>-L "$(OutputPathAbs.TrimEnd('\'))" -L "$(OutputPathAbs)..\v1.0" -L "$(OutputPathAbs)..\v1.0\Facades"</_LibDirs>
+      <_LibDirs>-L "$(OutputPathAbs.TrimEnd('\'))"</_LibDirs>
+      <_LibDirs Condition=" '$(TargetFramework)' != 'netcoreapp3.1' ">$(_LibDirs) -L "$(OutputPathAbs)..\v1.0" -L "$(OutputPathAbs)..\v1.0\Facades"</_LibDirs>
+      <_LibDirs Condition=" '$(TargetFramework)' == 'netcoreapp3.1' ">$(_LibDirs) -L "$(_CorlibDir.TrimEnd('\'))"</_LibDirs>
       <_Out>-o "$(MSBuildProjectDirectory)\$(IntermediateOutputPath)jcw\src"</_Out>
     </PropertyGroup>
     <Exec

src/Mono.Android/Android.Runtime/JNIEnv.cs

@@ -170,7 +170,9 @@ internal static unsafe void Initialize (JnienvInitializeArgs* args)
 			else
 				IdentityHash = v => v;
 
+#if MONOANDROID1_0
 			Mono.SystemDependencyProvider.Initialize ();
+#endif
 
 			BoundExceptionType = (BoundExceptionType)args->ioExceptionType;
 			androidRuntime = new AndroidRuntime (args->env, args->javaVm, androidSdkVersion > 10, args->grefLoader, args->Loader_loadClass, args->jniAddNativeMethodRegistrationAttributePresent != 0);

src/Mono.Android/Mono.Android.csproj

@@ -1,10 +1,11 @@
 <?xml version="1.0" encoding="utf-8"?>
-<Project Sdk="Microsoft.NET.Sdk">
+<Project>
 
+  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
   <Import Project="..\..\Configuration.props" />
 
   <PropertyGroup>
-    <TargetFramework>monoandroid10</TargetFramework>
+    <TargetFrameworks>monoandroid10;netcoreapp3.1</TargetFrameworks>
     <RootNamespace>Android</RootNamespace>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
@@ -30,6 +31,11 @@
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
   </PropertyGroup>
 
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'netcoreapp3.1' ">
+    <OutputPath>$(XAInstallPrefix)xbuild-frameworks\Xamarin.Android.App\$(TargetFramework)\</OutputPath>
+    <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
+  </PropertyGroup>
+
   <ItemGroup Condition=" '$(TargetFramework)' == 'monoandroid10' ">
     <Reference Include="mscorlib">
       <HintPath>$(OutputPath)..\v1.0\mscorlib.dll</HintPath>
@@ -73,6 +79,10 @@
     </Reference>
   </ItemGroup>
 
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netcoreapp3.1' ">
+    <ProjectReference Include="..\..\external\Java.Interop\src\Java.Interop\Java.Interop.csproj" />
+  </ItemGroup>
+
   <ItemGroup>
     <Compile Include="..\..\external\Java.Interop\src\Java.Interop.Tools.TypeNameMappings\Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs">
       <Link>JavaNativeTypeManager.cs</Link>
@@ -331,9 +341,16 @@
     <ProjectReference Include="..\..\build-tools\api-merge\api-merge.csproj" ReferenceOutputAssembly="false" />
     <ProjectReference Include="..\..\build-tools\api-xml-adjuster\api-xml-adjuster.csproj" ReferenceOutputAssembly="false" />
     <ProjectReference Include="..\..\build-tools\jnienv-gen\jnienv-gen.csproj" ReferenceOutputAssembly="false" />
-    <ProjectReference Include="..\..\external\Java.Interop\tools\generator\generator.csproj" ReferenceOutputAssembly="false" SkipGetTargetFrameworkProperties="true" />
-    <ProjectReference Include="..\..\external\Java.Interop\tools\jcw-gen\jcw-gen.csproj" ReferenceOutputAssembly="false" SkipGetTargetFrameworkProperties="true" />
+    <!-- Explicitly pass the target framework of the project so we don't have conflicts with the multiple targets in this file. -->
+    <ProjectReference Include="..\..\external\Java.Interop\tools\generator\generator.csproj" ReferenceOutputAssembly="false" SkipGetTargetFrameworkProperties="true" AdditionalProperties="TargetFramework=net472"/>
+    <ProjectReference Include="..\..\external\Java.Interop\tools\jcw-gen\jcw-gen.csproj" ReferenceOutputAssembly="false" SkipGetTargetFrameworkProperties="true" AdditionalProperties="TargetFramework=net472" />
     <ProjectReference Include="..\..\src\java-runtime\java-runtime.csproj" ReferenceOutputAssembly="false" />
   </ItemGroup>
 
+  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
+  <!-- Only build the 'netcoreapp3.1' version of 'Mono.Android.dll' once for the latest stable Android version. -->
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'netcoreapp3.1' And '$(AndroidFrameworkVersion)' != '$(AndroidLatestStableFrameworkVersion)' ">
+    <BuildDependsOn></BuildDependsOn>
+  </PropertyGroup>
+
 </Project>

src/Mono.Android/Mono.Android.targets

@@ -10,6 +10,7 @@
   </ItemGroup>
   <Target Name="_BuildJavaInterop"
       BeforeTargets="BeforeResolveReferences"
+      Condition=" '$(TargetFramework)' == 'monoandroid10' "
       Inputs="$(MSBuildThisFile);$(JavaInteropFullPath)\src\Java.Interop\Java.Interop.csproj"
       Outputs="$(OutputPath)\..\v1.0\Java.Interop.dll">
     <PropertyGroup>
@@ -134,6 +135,7 @@
   </ItemGroup>
   <Target Name="_GenerateFrameworkList"
       BeforeTargets="GetTargetFrameworkProperties;GetReferenceAssemblyPaths;ResolveReferences"
+      Condition=" '$(TargetFramework)' == 'monoandroid10' "
       Inputs="$(MSBuildProjectFullPath)"
       Outputs="$(OutputPath)RedistList\FrameworkList.xml">
    <MakeDir Directories="$(OutputPath)RedistList" />
@@ -185,7 +187,7 @@
   <Target
       Name="_CheckApiCompatibility"
       Condition=" '$(DisableApiCompatibilityCheck)' != 'True' "
-      AfterTargets="Build"
+      AfterTargets="CopyFilesToOutputDirectory"
       Inputs="$(TargetPath);@(ApiCompatibilityFiles)"
       Outputs="$(IntermediateOutputPath)CheckApiCompatibility.stamp">
     <CheckApiCompatibility