stagefright: Handle acquireWakeLock failure case

When the power service is not running, acquireWakeLock fails, but
TimedEventQueue still keeps a reference count to a non-existent
wakelock. This causes an assertion failure when trying to free the
wakelock. Only increment the reference count and save the wakelock
reference on successful acquire.

CRs-Fixed: 640796
Change-Id: I441bf8d4c3520eb6ce8a378d2744913de0a44f55