Add ModernPermissionManagerServiceImpl skeleton and two methods.

Also allow compatibility layer to talk to policy directly by
introducing scopes for getting and mutating state, because
compatibility layer shouldn't have direct access to states, while
scope also allows getting rid of a lot of boilerplate for passing
states around.

Bug: 182523293
Test: presubmit
Change-Id: I36f53383c261b51372178d8c183c190bdb3c7132