AAPT2: Merge-only mode for building static libraries

Add a new option for skipping resource references validation when
building a static library. This pushes resource reference validation
to the final link step (creating an installable APK), but allows a lib
to be linked without having the static libs of its dependencies.

More context: go/autonamespace-transform

Test: MergeOnlyTest + manually verified
Bug: 128824820
Change-Id: I1f3e3b7715a5244b8633c85519d94334c76ff664