Stop rejecting valid flings in quick contacts

I continue to regret basing MultiShrinkScroller off of
ScrollableViewGroup instead of directly basing it off of the
less buggy ScrollView code.

* Previously we only tracked velocity inside onTouch.
  Now, we will also track it in onInterceptTouchEvent, similarly
  to ViewDragHelper. This will result in double tracking of
  some events. This isn't a problem: VelocityTracker.cpp considers
  the timestamp when performing the velocity calculation.
* Don't dismiss vertical flings simply because they also contain
  a horizontal fling

Bug: 17919260
Change-Id: Ic0e512633dd6d99b1e55adfb56a57e4f170f2e73