[Xamarin.Android.Build.Tasks] incremental build improvements

[jonathanpeppers]

Fixes: #2247

We currently have two targets that fail to build incrementally:

• _ResolveLibraryProjectImports
• _GeneratePackageManagerJava

Both of these targets declare Outputs, but don't always update the
timestamp of the output file. They used CopyIfChanged logic, so they
won't write to the file if nothing changed.

However...

We can't just add <Touch/> calls here either, as the Outputs of
these targets are Inputs to other slow targets (_CompileJava,
_UpdateAndroidResgen, and friends). If we used <Touch/> here,
things would be worse and trigger the slow targets to run.

So the only option here is to use a "stamp" file, this allows these
targets to properly build incrementally and not trigger other targets.

I updated a IncrementalBuildTest to include a test case checking all
three of these targets.

Future changes:

• I am going to need to audit many of our MSBuild targets for this
  same problem, and so I will likely need to add many new stamp files.
• To simplify this, we should adopt a new convention of placing stamp
  files in $(IntermediateOutputPath)stamp\ or
  $(_AndroidStampDirectory).
• The stamp file should be named the same as the target where it is
  used as an Output.

Documentation:

I also started some initial documentation on "MSBuild Best Practices",
which we can expand upon in future PRs. See the Stamp Files section
on what is relevant in this PR.

[garuma]

🎉 ❤️