Honor crop bounds for long screenshot bitmap export

Make toBitmap take a rect of which parts we want to render. Enable the
CropView and use its selected bounds to derive that rect.

Bug: 179175127
Test: Take long screenshot, move crop bounds, edit and verify that
      bounds are matched.
Change-Id: I546ae4a7e8e39d7e9b88eb1f503b2d1bde31a3c7