Can now swipe away to dismiss second task

Previously we only allowed dragging the forefront task. Now you can
swipe up to dismiss the second task as well, but can't drag it down to
launch.

Also cleaned up page-scroll-while-dismissing logic to ensure it works
correctly in RTL and for new cases with dismissing adjacent pages.

Bug: 73187449
Change-Id: I1fe873c4cf1380b951dd3b2e396ab401ca1f7470