Introduce zen mode.

 - New global zen mode setting with three values:
   ZEN_MODE_OFF, ZEN_MODE_LIMITED, ZEN_MODE_FULL
 - Status bar icon (on lhs)  when in _LIMITED / _FULL
 - Rules are hardcoded for now:
   _LIMITED = intercept all notifications except calls/alarms
   _FULL = intercept everything
 - UI affordance in shade to set mode
 - Only one exit condition: "Until you delete this"
 - Intercepted notifications:
   - appear darker in the shade
   - do not cause sound/vibration
   - do not fire fullScreenIntents
   - do not contribute an icon to the status bar

Change-Id: Ic36c95982b483432006bf9dab102a31370382aa9