Signature policy for overlayable items

Add encoding/decoding of new policy for overlays. Signature enforces
that an overlay package is signed with the same key as the actor of
the target resource, so that an overlay can be installed by the user
as a normal app but restricted to those built by the author of the
actor (which can be the same as the target).

This also enforces that a valid policy is specified.

This doesn't implement the actors nor the signature check.

Bug: 119402606

Test: ResourceParserTest ParseOverlayablePolicy
Test: ProtoSerializerTest SerializeAndDeserializeOverlayable
Test: aapt2_tests

Change-Id: I8495ad790c2ebd51759bc6eba81149680c209475