Fix incremental builds with AAPT2

When we removed the src/ directory, the R.stamp file was removed -- with
AAPT, this triggers src/android/Manifest.java to be regenerated. But
AAPT2 only uses the R.stamp file as a timestamp to find the
Manifest.java file, not to create it.

So add a new CleanSpec.mk to fix the builds by removing the target that
AAPT2 uses to generate the src/android/Manifest.java file.

Test: None
Change-Id: If56f326ad701f7f92aa7c8aeda57e92d9c62cff9