APIs for registering config activation broadcasts

Creates a StatsManager API to register a pending intent that is invoked
whenever the active status of any of any config sent by that uid
changes.

ag/6112554 and further cls are needed to properly send back the pending
intent.

Bug: 123372077
Test: build success on blueline-eng. More thorough testing should be
done once ag/6112554 is submitted.

Change-Id: I36985c9820d4e7ac178064924190af88ea822254