stagefright: fix TimedEventQueue wakelock

If an event is taking a wakelock, the wakelock must be
released after the event is fired.
If the wakelock is released before and the event execution
implies some sleeps or I/O the system can go idle as
no wakelock is held anymore.

Bug: 11976087.

Change-Id: Ie7df8ed4834952ff818ff27d6be415c0b1794a9f