QuestionFragment: Add some error checking.
ItemsContentProvider: Get more items when necessary.

This was a regression after the change to use AsyncTask.
Fragments: Use restartLoader() instead of initLoader().

This will let us get new data from the query, for instance to get the new
"next" item.
QuestionFragment: onAnswerButtonClicked(): Reset the question ID to null too.
ClassifyFragment: addChildFragments(): Change to addOrUpdateChildFragments().

And update existing fragments if they have already been added.
QuestionFragment: Let ClassifyActivity show the next item when finished.

Add a callback that ClassifyActivity must implement, as a parent of
QuestionFragment. Then set the ID to the "next" again, and update
the fragment, instead of just closing the activity.