Fixed an issue where the top edge flickered

Because the overscroller is going over the edge, scrolling
could sometimes become negative. Also, the edge position
didn't actually match the position of the overscroll.
Now the overscrolling is done via the stackY, which also
updates the qsEdge.

Fixes: 191246064
Test: overscroll with notifications, observe no flickering
Change-Id: Iea994b05152f1c2465224b5371b4330a67310827