Fix NPE.

When entering the notification prefs sub activity without the increasing
ring pref being visible and leaving it right away, onActivityStop()
threw an NPE because the handler wasn't yet constructed.

Change-Id: Ia4291d8ac3d1dbf76d432db7e0e973844163c4ae
Merge remote-tracking branch 'cm/cm-12.0' into cm-12.0