Bug 935273: don't decode full-size image during pick activity unless necessary

Modify ImageEditor to have a "crop only" mode where it displays
only a preview image and crop handles and does not do other image editing.
In this mode it does not decode the full-size image unless the user actually
crops the image.

Address review comments; handle orientation change; handle case where image has no preview
Merge pull request #13962 from davidflanagan/bug935273

Bug 935273: wip patch for testing