AAPT2: Propagate SPEC_OVERLAYABLE flag to final APK

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