[Xamarin.Android.Build.Tasks] aapt2, move ConvertResourcesCases to a new target

[jonathanpeppers]

Fixes: #2356

When $(AndroidUseAapt2)=True, if you look for a log entry of a file
that is processed:

Processing: C:\Users\myuser\Desktop\Git\xamarin-android\tests\Xamarin.Forms-Performance-Integration\Droid\obj\Debug\90\lp\22\jl\res\values\values.xml   10/30/2018 4:28:18 PM > 1/1/0001 12:00:00 AM

You will see this repeated several times (9 in case of a Xamarin.Forms
project).

It turns out, since the _CompileAndroidLibraryResources target is
batched, it is called once per item of
@(_LibraryResourceHashDirectories):

<Target Name="_CompileAndroidLibraryResources" DependsOnTargets="_CollectLibraryResourceDirectories"
        Condition="'$(_AndroidUseAapt2)' == 'True'"
        Inputs="%(_LibraryResourceHashDirectories.FileFound)"
        Outputs="$(_AndroidLibraryFlatArchivesDirectory)%(_LibraryResourceHashDirectories.Hash).stamp">
...
<ConvertResourcesCases ... />
<Aapt2Compile ... />
...
</Target>

It calls ConvertResourcesCases for each call, which is not ideal.
ConvertResourcesCases operates on the whole tree of resources, so it
really should just run once: before <Aapt2Compile/>.

I think the fix here is:

• Move ConvertResourcesCases to a new
  _ConvertLibraryResourcesCases target
• The new target's Inputs should be
  @(_LibraryResourceHashDirectories->'%(FileFound)') and not batched.
• The new target's Outputs should be a new stamp file following our
  new convention.
• The new stamp file should also be used as
  AndroidConversionFlagFile.
• _CompileAndroidLibraryResources now depends on
  _ConvertLibraryResourcesCases, and it should only run on the first
  batched instance of _CompileAndroidLibraryResources.

[dellis1972]

@jonathanpeppers looks ok, do you have any data on how this effects the build times for Aapt2?

[jonathanpeppers]

This won't help as much for "Hello World" since #2348.

So I timed the Xamarin.Forms Control Gallery:

Before:
500 ms  ConvertResourcesCases                     55 calls
After:
136 ms  ConvertResourcesCases                      9 calls

So this is going to help more with larger projects. #2348 already helped this issue alot.

Logs: logs.zip