Add staging-public-group to aapt2

staging-public-group is a tag for putting resources that have been
added during platform development, but have not yet been finalized,
into a separate resource id namespace.

R.java fields of staged resources are non-final, so when the SDK is
finalized, applications using the android R.java will automatically
use the new finalized resource id without having to recompile.

Staged resources can exist either in the same type id as the type's
non-staged counterpart or in a separate type id. Multiple
staging-public-group tags each with a different type id can exist
simultaneously, which allows for multiple versions of the platform
to be developed at once.

Bug: 183411093
Test: aapt2_tests

Change-Id: Ibb6c84c3626751e33c6097f35a03e306bb85616a