Optimization: cache transformed images.
Transformed images are now stored in the same DiskLruCache as the response cache.
Prior to this change, transformed images were always retransformed from the original image
on cache hit.
This change has the benefit of allowing the original image (which may be very large)
to stagnate and drop off the cache, while allowing the transformed image to persist.
It also speeds up load times by not having to perform the transformation every reload.

Change-Id: I084b78ca0d94721009ccbc2dcffbe315b2e6c44a