Sort notifications by importance.

- Importance trumps 'package priority'
- Only notifications with a high enough importance can be intrusive
- default and lower importance notifications don't appear on the lock
screen and appear below the speed bump in the shade

Also, stop downgrading MAX importance noisy notifications.

Bug: 26425560
Change-Id: Ibb952eb6f8d0d336a1060332f58c8ba622f29867