ItemsContentProvider: Add comments about reusing old cache files.

Though this only happens when doing hacky things while debugging.
Remove unnecessary imports.
QuestionFragment.saveClassification(): Prevent saving with the 'next' ID.

Though it should never get this far anyway.
ClassifyFragment: remove a now-irrelevant comment.
ClassifyFragment: Really don't show UI when there is no next item.
ClassifyFragment: Warn when there is no next item.
strings.xml: Add a full stop (period) at end of messages.

I prefer it. I can't find any specific Android design guideline about it.
German translation: Add the new strings, though they are not really translated.