Singleton: Add localeIsDifferent() so we can compare on configuration changes.
ClassifyFragment: showLoadingView(): Hide the child fragments when showing the loading view.

Because they should never appear together.
QuestionHelpFragment: Remove mSingleton that shadows the one in the base class.

This led to some unexpected null Singletons.
ItemFragment: Add initializeSingleton() to avoid some copy/pasted code.
QuestionFragment.update(): Don't continue if we are waiting for the Singleton.
fix
Singleton: Keep a list of callbacks instead of just one.

Otherwise, subsequent calls wipe out the previous callback, so it is
never called.
Singleton: Avoid calling the constructor twice just to discard one.