Add service for applying per-app compat overrides delivered via Device Config.

This CL only adds the listener on device config changes, package
added/changed/removed support will be added in a follow up.

Bug: 190024878
Test: atest FrameworksMockingServicesTests:AppCompatOverridesServiceTest
Test: atest FrameworksMockingServicesTests:AppCompatOverridesParserTest
Change-Id: Ifda798abca5ed97cd4699dfd1244053ab2796586