Snap to last task if swiping down at a greater Y than X velocity.

If you swipe down slightly diagonally, it will almost always be treated as a swipe to the next task rather than a return to the current task. If we are swiping down and the Y velocity is grater, we should snap to the current task instead.

Test: Manually in fully gestural mode.
Bug: 222117127
Change-Id: I0f591fa5117291a9c095c2b733280464d29103d9