Improve drag-to-remove detection

Instead of checking the position of the finger during the drag gesture,
we now check if the bounding box of the item being dragged contains the
center of the remove button. This makes dragging to remove much easier,
especially for larger widgets.

Test: manually by dragging widgets to the remove button
Flag: EXEMPT bugfix
Fixes: 319134429
Change-Id: I42658889e301ed84c4a511fc14abcd332e5271c3