Facilitate subclasses of SingleTypeAdapter that wish to implement android.widget.Filterable

This exposes a new method, getItems() as protected. 
Primarily, this allows subclasses to implement android.widget.Filterable more cleanly. The subclass can cache a list of all items. Please let me know if I'm missing something and there's a better way to do this.

I didn't apply this change to any other adapters.