added Android Spinner support with SpinnerViewProvider

I added Spinner "entries" property support as I wanted to be able to give entries for a Spinner from my ViewModels. 

ViewModel:

 public IObservableCollection<String> testList = new ArrayListObservable<String>(
            String.class,
            new String[] {new String("A"), new String("C")}
    );

View:

 <Spinner android:layout_width="match_parent"
             android:layout_height="match_parent"
             android:id="@+id/testListSpinner"
             binding:entries="testList" ></Spinner>