Make Notification settings a proper PreferenceFragment with pager

[konklone]

We can:

• [x] Remove the Android support library and use the native Fragment/etc classes
• [ ] Make the NotificationSettings PreferenceActivity a PreferenceFragment
• [ ] Make NotificationTabs a pager instead, like the others, that loads fragments

[konklone]

Here's a screen from Google Play for 2014-04-23, showing that ~20% of "Current Installs by Device" are Android 2.3.X:

But, Google Analytics suggests that actual usage (% of actual sessions observed over the last 30 days) is more like 3%:

Google Analytics is likelier to be correct.