Fix race condition in CarrierTextController

If WakefulnessLifcecycle dispatches to CarrierTextController after the
CarrierTextCallback has been cleared but the observer has not been
removed, there will be an NPE.

This change guards the callback against dispatching to a null object.

Test: build and unlock
Fixes: 142130321
Change-Id: I5da46a0d5c8308870f2b67ae4622daf8e2d53b2f