ItemsContentProvider: Check some File return values.
QuestoinFragment: onOptionsItemSelected(): Don't let a switch/case fall through.
ZooniverseClient.generateAuthorizationHeader(): Specify an encoding to String.getBytes().

Because that is more robust.
Avoid repeating the "UTF-8" string throughout the code.
ItemsContentProvider: getStreamTypes(): Return a clone of the String array.

Instead of the String array itself, because that allows the caller to
change implementation details.
Found by FindBugs.
ItemsContentProvider: Make some inner classes private and static.
QuestionFragment: Add a comment about padding.
Layout XML: Add start/end where we use left/right.

To support both old Android versions and RTL on newer Android versions.