Update CellLayout.DEBUG_VISUALIZE_OCCUPIED to include drag over targets

Instead of just drawing the occupied cells in red, now we draw the occupied cells based on drag over regions:
- Dragging over the red regions will reorder the cell
- Dragging over the green regions will create/add to a folder

Test: visual with internal flag on
Bug: 204406063
Change-Id: I62105c1c1a1101b6cd6f9fd222980d03ba6d8b84