Improve fling physics for recents

- Don't limit swipe out velocity
- Make sure we pick up all touch events in VelocityTracker
- When already scrolling and touching again, don't reset scroll
  state, so we don't wait for the touch slop again to start
  scrolling (this makes it more consistent with ScrollView).

Bug: 18536759
Change-Id: I1d43c77db78beec6d0b8c949e67141a2665de892