Actor signature overlayable policy

There are cases where an app can ship overlays for itself,
but the "signature" policy as described would open up
a vulnerability by allowing the system actor to create
and sign any arbitrary overlay that will apply to the target.

To prevent this, redefine "signature" as target package only,
and introduce "actor" for checking against the actor signature.
Any app that wishes to use both can include both policies.

Bug: 130563563

Test: m aapt2_tests idmapt2_tests and run from host test output
Test: atest libandroidfw_tests

Change-Id: I1c583a5b37f4abbeb18fc6a35c502377d8977a41