Improve performance when changing "Unstable Updates" preference

This is only a partial solution to #520.

It does as I suggested in the issue comments, by doing less work when the preference is checked. The proper solution is an IntentService which queues requests to update these details, but that is a (slightly) larger change for the future.

I also noticed it wasn't correctly notifying the UI of the change, so this now notifies the list of apps which can be updated. That was hard to test though, so not sure if it updates the UI correctly or not. It shouldn't do it incorrectly, but it may not work. The reason it should work is because the AppListFragment (baseclass of CanUpdateAppsFragment) uses a cursor loader with the AppProvider.getCanUpdateUri() URI. This CursorLoader should automatically attach an observer for that URI and requery if required.