Multi touch should scale the image below, not the crop overlay

Ideally, we pinch-zoom in/out the image and only change the bounds of overlay by dragging the corners. Multitouch on the overlay doesn't provide much value in most cases.

I am currently working on implementing this by changing the ImageView to a PhotoView(chrisbanes). I'll give a pull request when I have it ready.

Start with the ImageView and the crop overlay touch handling described in the issue, then review how PhotoView (chrisbanes) would fit the image interaction. Done means multitouch pinch-zooms the image while dragging overlay corners changes only the crop bounds; verify both interactions on Android.

Written by the indexing model from the issue text.