It's true that MessageNotifier.updateNotification() is called from a number of places, which might be the strength of adding the check in the method itself.  Is there ever a case when we'd want notifications to display if the app isn't the default SMS app?  

It seems to me that we want to essentially disable notifications.  We might even want to show the "enable notifications" preference disabled and grey'd out if the app isn't the default app on >= kitkat.