There might having windowless dream service like doze dream which won't
hava attached activity, so call finish() directly in onWakeUp() since
finish() itself deals with finishAndRemoveTask() as well.
Fix: b/150110220
Test: manual test, won't throw NPE while leaving doze service.
Change-Id: I982c2f6fb1859e869732ef8795b61bf96e8cff64
Merged-In: I982c2f6fb1859e869732ef8795b61bf96e8cff64
(cherry picked from commit 45f0d572c1)