Fix small issues with event flow for quickscrub

- Ensure that we return whether we handle quickscrub fling so that we
  don't proxy the associated up event
- Ensure that if we have already started quickscrub, that we don't return
  false to continue proxying events midway

Bug: 67957962
Bug: 70180755
Test: Enable quickscrub, start scrubbing and swipe up

Change-Id: I9e3728ed3781b36c38fc16c0d047665fb0c3b904
Signed-off-by: Winson Chung <winsonc@google.com>