Resources can be marked as overlayable, which means they can
be overlaid by runtime resource overlays.
This change propagates this state to the final resource table that
is installed on device.
Future work:
- Have the idmap tool respect the overlayable state and ignore
entries that overlay anything else.
Bug: 64980941
Test: make aapt2_tests
Change-Id: Id45b1e141a281be2ee32a4ac3096fcf1114d523b
71be70507d