DeskClock: use low priority channel for snooze notification

* Chances are the user has just completed a back-breaking maneuver
  to find and snooze their extremely loud and annoying alarm. There's
  no need to alert them again. Let the pour user sleep.

* Make it ongoing too, to prevent accidental swiping away of the
  snoozed alarm notification which may happen due to side effects of
  PASD (post-alarm stress disorder).

Change-Id: Id949cff72905c5226ade6b05abeca4c8d0a541c2