Making ScreenOnUnblocked and ScreenOffUnblocked public

This is needed because we soon plan to introduce the concept of
DisplayStateNotifier, which will need to interact with this class, and
hence would be neater to move it outside of DisplayPowerController
Bug: 259411586
Test: atest com.android.server.display

Change-Id: Ib353f051fcb76111688ff8ca0e65846a98c21041