heads up notifications, v0.1

1. re-name intruder alerts to heads up notifications
2. use interrupt for the verb form, instead of intrude
3. new policy: no full screen, high priority, screen on, not locked or dreaming
4. controlled by global setting, with an observer
5. only content view for now, expandable is a todo

to turn it on:
  adb  shell settings put global heads_up_enabled 1
calendar notifications (not pop-up) work well.

Change-Id: I253418c217d0a5cf81dc2fa001f4bad90fafcce5