scaleType can not be set when zoomable is set to false

Creating a `PhotoView` and setting the zoomable property to false works as expected but it is not possible to change the scaleType anymore. It has no affect and is always FIT_CENTER.

Diving into the attacher it becomes obvious that the update method resets the scaleType when zoomable is false and does not update the base matrix with the new scaleType. https://github.com/chrisbanes/PhotoView/blob/master/photoview/src/main/java/com/github/chrisbanes/photoview/PhotoViewAttacher.java#L513