AAPT2: Add -A (assets) support

Looks like the build system doesn't support assets/ for
resources, so we will re-introduce them in aapt2, even though
we're just copying them around and they would be better
suited for inclusion in the APK when classes.dex gets inserted.

Bug: 35461578
Test: CTS test android.content.res.cts.AssetManager#testAssetOperations should pass
Change-Id: I18361d7367d476806bcf7154ee76df3f0e83b565