Fix race condition in WakeLocks.

A previous fix to PowerManager.WakeLock introduced a new bug. When
a reference counted wakelock (such as SMSDispatcher) is acquired
multiple times in quick succession with a timeout, the first timeout
would remove the second callback from the handler, decrement the
reference count, but then the wakelock never gets released because
the second callback has been removed.

Bug: 4535237
Change-Id: I4f1240189bf3ed3396c86200c687282597b107d6