Improve quick scrub

- Snap to the next task when quick scrub starts, but don't allow
  snapping to further pages until the transition to overview
  completes (to prevent overshooting)
- Simplify quick switch to just launch the task that was snapped
  to in onQuickScrubStart
- Cleanup some state code

Bug: 70180755
Bug: 74014237
Change-Id: I7a4a0f1a568947b1f5e56a27d7328e47b05a675d