Don't accidentally scroll QC off screen.

Behavior changes
1) Halt fling once it hits the top of the window
2) Add top EdgeEffect
3) Add springyness

These changes result in much simpler nested-scrolling
calculations.

UX may want to change some of the constants
after playing with this for a day. However, the
logic changes should remain unchanged.

Bug: 18571742
Change-Id: Icbc063beb2061bf7cd1e4401b16e604415140242