Ignore the system uid to avoid deadlocks

The ActivityManagerService can acquire or release wake locks with the
ActivityManagerService locked.  Now that the PowerManagerService
notifies it of wakelocks, this can cause a deadlock.  Prevent this
situation by ignoring wakelocks from the system uid, which should be
never be killed anyway.

The deadlock occurs when the ActivityManagerService is locked and
the WakeLock acquire/release locks PowerManagerService.mLocks.  Another
thread may have locked them in a different order at the same time.

Signed-off-by: Christopher Lais <chris+android@zenthought.org>