Standardize quickstep velocities

- Compute velocities as px/ms from VelocityTracker, instead of
  computing px/s and dividing by 1000 in various places
- Use quickstep_fling_threshold_speed in TriggerSwipeUpTouchTracker
  instead of ViewConfiguration#getScaledMinimumFlingVelocity
- Check MotionPauseDetector to determine whether a fling is
  sufficient to escape after pause, in additon to above threshold

Bug: 159089437
Change-Id: Ib0fae7804e1f008f3b6ab37d6c463352c224b0e4