Encoding of <overlayable> and <policy>

This change defines two new chunks for encoding overlayable information.
RES_TABLE_OVERLAYABLE_POLICY_TYPE contains flags that represent
restrictions enforced on overlays that try to overlay a specific set of
resource ids. The chunk header is followed by ResTable_ref for each id
that belongs to the policy type. A policy chunk will be created for
every unique combination of policies that are defined in overlayable
declarations.

RES_TABLE_OVERLAYABLE_TYPE holds policy blocks. Since <overlayable>
does not currently have any attributes, only one overlayable block is
encoded in an APK.

This change also removes the SPEC_OVERLAYABLE flag because the runtime
does not use the flag, and the overlayable chunk encoding renders it
obsolete.

Bug: 110869880
Bug: 117545186
Test: libandroidfw_tests and aapt2_tests
Change-Id: I45ae9bf4176699f14c85e2b7a2e8560185d8a0b8