[Xamarin.Android.Build.Tasks] <GenerateJavaStubs/> improvements

[jonathanpeppers]

I went through the _GenerateJavaStubs MSBuild target and the
<GenerateJavaStubs/> MSBuild task. There was an "easy" win to
improve incremental build performance. I also made a few other general
improvements to <GenerateJavaStubs/>.

Inputs & Outputs

_GenerateJavaStubs has Inputs of:

Inputs="$(MSBuildAllProjects);@(_ResolvedAssemblies);$(_AndroidManifestAbs);$(_AndroidBuildPropertiesCache);@(_AndroidResourceDest)"

Lets say a NetStandard assembly changes, such as a XAML change in a
Xamarin.Forms app. In this case _GenerateJavaStubs does not actually
need to run again.

We can use @(_ResolvedUserMonoAndroidAssemblies) as an input instead
of @(_ResolvedAssemblies).

This is the biggest win--to skip this target completely.

selectedWhitelistAssemblies

ManifestDocument had the concept of a "whitelist" assembly, which
currently consisted of a single assembly:

internal static readonly string [] FrameworkAttributeLookupTargets = {"Mono.Android.GoogleMaps.dll"};

Mono.Android.GoogleMaps.dll no longer exists, so I was able to
remove a bit of code that still used that dead codepath.

Typemaps

Just a couple minor improvements here:

• Fixed the usage of the [Obsolete] constructor for
  TypeNameMapGenerator. Using a (TaskLevel level, string value)
  callback instead.
• I reused the MemoryStream and got rid of the UpdateWhenChanged
  method.

Tests

I updated the IncrementalBuildTest.ProduceReferenceAssembly test
quite a bit:

• The library project using $(ProduceReferenceAssembly) is now a
  NetStandard library.
• I check all the targets that should be skipped from now on.

I made a few changes in Xamarin.ProjectTools to clean things up:

• The DotNetStandard class should just define
  Language=XamarinAndroidProjectLanguage.CSharp by default.
• Added a ShouldRestorePackageReferences property for deciding if we
  pass /restore or not.
• The DotNetStandard class always needs to use /restore, even if
  there are no PackageReferences.
• I moved a couple methods from DotNetXamarinProject to
  XamarinProject so the DotNetStandard has them.

Results

Testing the Xamarin.Forms project in this repo. A fresh build I wasn't
able to see a noticeable improvement.

However, an incremental build with XAML-change:

Before:
  861 ms  _GenerateJavaStubs                         1 calls
After:
  n/a

The target is skipped now:

_GenerateJavaStubs:
Skipping target "_GenerateJavaStubs" because all output files are up-to-date with respect to the input files.

Overall this seems to be ~800ms better for incremental builds with
XAML changes.

[jonpryor]

It doesn't immediately make sense, but this change seems to have broken ~every on-device .apk test: https://jenkins.mono-project.com/job/xamarin-android-pr-pipeline-release/518/testReport/ e.g. https://jenkins.mono-project.com/job/xamarin-android-pr-pipeline-release/518/testReport/Xamarin.Android/Locale_Tests/Possible_Crash___Release/

Unhandled Exception:
System.ArgumentException: Handle must be valid.
Parameter name: instance
  at Java.Interop.JniEnvironment+InstanceMethods.CallObjectMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method)
  at Android.Runtime.JNIEnv.CallObjectMethod (System.IntPtr jobject, System.IntPtr jmethod)
  at Java.Interop.TypeManager.GetClassName (System.IntPtr class_ptr)
  at Java.Interop.TypeManager.CreateInstance (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer, System.Type targetType)
  at Java.Lang.Object.GetObject (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer, System.Type type)
  at Java.Lang.Object._GetObject[T] (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
  at Java.Lang.Object.GetObject[T] (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
  at Java.Lang.Thread.get_DefaultUncaughtExceptionHandler ()
  at Android.Runtime.JNIEnv.Initialize (Android.Runtime.JnienvInitializeArgs* args)

Looks like the typemap.* files are missing a type?

[jonathanpeppers]

I think the macOS failures are OK:

Test Result (2 failures / +1)
JnienvTest.Mono.Android_Tests, Java.InteropTests.JnienvTest.DoNotLeakWeakReferences / 
ReleaseXamarin.Android.Bcl_Tests.nunit.Possible Crash / Release

It seems like the "adb crashed" thing happened.