DeskClock: introduce low priority alarm notification channel

* Low priority alarms such as an upcoming alarm or snooze
  notification shouldn't make noise, vibrate or be intrusive.

* In addition, set them as ongoing notifications (preventing a
  user from swiping them away) since they should have to
  intentionally cancel the alarm.

Change-Id: I84ba2919dce39ecd4b002f501a397e2edf8ebd3c