[Xamarin.Android.Build.Tasks] fix incremental builds w/ native assembler

[jonathanpeppers]

When testing performance, I found an issue with _GenerateJavaStubs:

1. Change MainActivity.cs in an Android app project and hit F5.
2. The Build runs _GenerateJavaStubs
3. The Install runs _GenerateJavaStubs
4. Any subsequent build (even with no changes) will run
   _GenerateJavaStubs...

The log reads:

Building target "_GenerateJavaStubs" completely.
Input file "obj\Debug\90\android\assets\App14.dll" is newer than output file "obj\Debug\90\android\typemap.jm.arm64-v8a.s".

In 74cf0ec, we modified the Outputs to be:

Outputs="$(_AndroidStampDirectory)_GenerateJavaStubs.stamp;@(_TypeMapAssemblySource)"

This would be OK, except in some cases we don't update this file:

MonoAndroidHelper.CopyIfStreamChanged (ms, asmFileName);

If _GenerateJavaStubs runs, but the typemap doesn't change, we will
be stuck in a spot where _GenerateJavaStubs will always run on
incremental builds.

_GeneratePackageManagerJava has the exact same problem as well.

IncrementalClean

The real issue 74cf0ec was trying to solve was that some of these
typemap.*.s files were getting deleted during incremental builds.

A PR build was hitting:

_CompileNativeAssemblySources:
    Assembler messages:
    can't open typemap.mj.armeabi-v7a.s for reading: No such file or directory

Due to:

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

We suspected IncrementalClean was deleting these files, but we
weren't able to find the right log file where that happened...

Going through things again, I figured out what was going on:

• @(_EnvironmentAssemblySource) and @(_NativeAssemblyTarget) were
  not added to FileWrites
• _CreateApplicationSharedLibraries was not running during Build
  but SignAndroidPackage. This means it was running after
  IncrementalClean. Any files it produced that were in FileWrites
  are ignored...
• We can run _CreateApplicationSharedLibraries right after
  _CompileJava. This has other benefits than correctness for
  IncrementalClean, such as when a build error occurs. The UX is a
  bit nicer in IDEs for errors from Build than Install.

After these changes I can remove the extra Outputs from
_GenerateJavaStubs and _GeneratePackageManagerJava and solely rely
on the stamp files as an output.

Moving _CreateApplicationSharedLibraries to Build should not
impact incremental build performance, as it should be target that is
skipped on almost all builds.

I also added a new test for these scenarios, verifying targets skip
and files are added to FileWrites appropriately.

Other fixes:

• AndroidEnvironment files were not an Input to
  _GeneratePackageManagerJava. So changing environment vars seems
  like it would always require a Rebuild to work...