API review feedback for ThemedSpinnerAdapter, Spinner

Moves themed interface out of Spinner and extends SpinnerAdapter, updates
Spinner constructor to take a Theme rather than a Context.

Does NOT change BaseAdapter to implement ThemedSpinnerAdapter, because
the BaseAdapter class does not have any notion of layout inflation and
that would break the contract implied by ThemedSpinnerAdapter.

Bug: 21571899
Change-Id: Id7e8d630458857ce6c93a6a8b8f920e169ee1152