Fix CropView sizing on full-width images

Full width images happen when the long screenshot isn't particularly
long, typically an error case.

- Ensure that ImageView and CropView take up the whole space defined by
constraints.
- Set padding on ImageView, factor that padding into the image size
computation for CropView.

Bug: 183278746
Test: Validate correct crop bounds display on a full height and a full
      width long screenshot.
Change-Id: I0f71ddc051f8cf6d295a348fe3cae3cbe00e4077