Fix DnD logic for determining current drag screen / panel

=> we want to use the extremum of finger location and object
   center for snapping to a new screen (max when snapping to right,
   min when snapping to left)
=> when selecting a panel within the current screen, we want to
   rely strictly on the object center to determine selection; there
   is plenty of space to be consistent here, and extremum logic
   could cause thrashing back and forth with no page movement.

Test: Manual. See videos in b/221104663. On two-panel and one-panel
      test dragging a widget between different screens with
      different starting positions of where the widget goes down.

Bug 221104663

Change-Id: I8f0f13f07b753752087a90c4bacb76cfbf7aa5bf