Add CropView for long screenshots

View is hidden for now since it doesn't do anything. Placeholder UI,
made up colors, etc.

Also make edit use the preferred screenshot editor (matching screenshot
behavior).

Bug: 148519342
Test: Make CropView visbible, interact with UI to verify it behaves
      properly.

Change-Id: Ib93616b7355b6b69fe556ff37a570d73af8844d1