DeskClock: Fix missed alarm notification flickering

Because when set alarm time to AlarmManager, two different type alarms
are created. Both of them will be triggered as the alarm time is up.
If alarm state is minus, the instance will be registered again and
another two different type alarms will be created. Then the process will
get into a cycle. E.g this cycle will cause missed alarm notification
flickering. So just do noting when AlarmStateManager receive state
change action to change state into POWER_OFF_ALARM_STATE.

Change-Id: I3024e6f035a1623f71711fffb458bd1c34c8de53