Add MagnifierView to long screenshot UI.

- Add a listener for CropView so MagnifierView can react to its motion
  events.
- Make CropView track deltas over the entire motion sequence instead of
  per-input (tiny deltas were causing floating precision drift)
- Animate the MagnifierView in and out.

Lots more to do including smarter view placement, better handling of
display at the edges, etc, but this seems like a good point to pause and
check in.

Bug: 179499370
Test: Invocation of long screenshot, observe magnification when altering
      crop boundaries.
Change-Id: Iae721afe835882f383b5f46f6159c06c4c2ebca4