AAPT2: support removed_ entries in groups

Ignores and increments the ID if the entry contains removed_, to prevent
shifting IDs by leaving a hole, allowing staging resources to be removed
before finalization.

This was broken out from the parent change to allow other CLs fixing the
resource IDs to be merged without splitting public.xml.

Bug: 222537368

Test: manual, see bug for other changes

Change-Id: I56f4fd70824bd42ec62f4fa0a2f067e2a3c094b5