PhotoView was not extensible

Defining IPhotoView almost allowed for a very flexible way of allowing
users to implement any PhotoView class on their own. (If they were not
able to inherit from PhotoView directly due to already having base
classes). This fix makes PhotoViewAttacher look for IPhotoView instead
of PhotoView for a few special cases, and made getDrawMatrix public.
This allows any user class to implement IPhotoView and behave correctly.
Fixed setZoomable not being FIT_CENTER

If you set your PhotoViewAttacher to setZoomable( false ) the image
would be not displayed as FIT_CENTER.