AAPT2: Allow empty group definitions

With ABI, screen density, and locale, it is possible to use a shorthand
notation when the group only has a single entry. The shorthand is to
leave the group empty and use a valid configuration for the group name.

Test: manually ran optimize command
Test: unit tests

Change-Id: If2d091e587474847c6c9e9be1a29196b261cc82d