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.
See merge request !414