Implement the floating cursor via the fisheye magnifier.

 - Remove the lineLeft/lineRight source bounds.
 - Make the magnifier center X can be away from the cursor X.
 - Let Magnifier know how to render floating cursor.

Test: locally verified.
Bug: 149883037

Change-Id: Iae3c68fa9cda0a83704fa30fa7b9f4db0a8d3314