Stop showing ADB and MTP/PTP/USB notifications at the top.

Two parts to this:
1. Stop treating FLAG_ONGOING_EVENT notifications specially
   (in particular, ordering them at the top of the panel).
2. Set the priority bits on the system UI notifications
   appropriately (low).

Change-Id: I3bde7e573654c5aad5e1c5d29e6a21ba94edcc5b