Refinements to notification controls in Settings.

- the switch is now a checkbox
- a dialog is posted to explain the ramifications of
  disabling notifications
- the user may no longer disable notifications from system
  apps (signed with the system cert); this is roughly the
  same as the constraint on disabling packages (with the
  exception that launchers may not be disabled entirely, but
  their notifications may still be disabled).

Bug: 6493120 (checkbox)
Bug: 6448988 (confirmation dialog & protect system apps)
Change-Id: I6c7a47ba2eb943936d7f59cfc807a4390acc980d