Finally, a place for notification settings.

Consolidated under the new Notifications settings are:

  - Ringtone (from Sound)
  - Zen Mode (from Sound)
  - Pulse LED (from Display)
  - Heads-Up Notifications (from Display)
  - Notification Access (from Security)
  - Show when locked (from Security)

Change-Id: I214d03ba1e356e088323a3058d98d390b8a8f988