Fix concurrent modification error in ShortcutService

The internal api ShortcutService#postValue isn't protected by mLock and
thus causing concurrent modification exception.

This CL temprorary fixes the issue by protecting above api with existing
lock. In the long run we should be moving the lock to package level,
which will be implemented in b/186011943

Bug: 187062117, 188397473, 188277633
Test: manual
Change-Id: I93537f8abe7a2ec2787949d109c6b3045664d8dd