Prevent NPE whlie leaving windowless dream service.

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