Settings: fix QuietHour start time

when user decide when quiethour will start/stop using time preference
alarm manager will setup the time, and call stop service in many times
before starting the service

this add some check to prevent call stop service before time to start

fix http://jira.omnirom.org/browse/OMNI-595

Change-Id: Ide4cb7f651ab153e1738d309992628b951e551c1