Make CropView handle images that aren't full-height

Compute the ImageView's extra top and bottom padding when it gets a
drawable or does layout, pass that information to CropView to adjust
its boundaries accordingly.

Also give MagnifierView some elevation so it doesn't get obscured by the
buttons.

Bug: 180967209
Test: Take a long screenshot at the top of the settings app, observe
    that the image isn't full height, ensure that crop handles properly
    honor image bounds and those bounds are honored on export as well.
Change-Id: Iac859995b362724a0db00b666a6ff970a0ccba1d