…new target
Fixes: dotnet#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`.