Combine ImageLoader and CropLoader utility classes.

Bug: 9356969
Bug: 9170644

- Refactor to eliminate code duplication in image loading classes.
- Fix bugs caused by the inability to determine MIME types for images
  that aren't in Gallery's content provider (causes b/9356969).

Change-Id: I30c2ba5c0cd37aac624681a2cec9abddaa78f9c2