Fixing an issue where you would inadvertently start fastscrolling.

- This would happen when you scroll near the thumb and pass through the
  area where the scrollbar was when the gesture started, and this CL
  prevents gestures that move beyond a fixed distance from the original
  touch position that is not already a fastscroll.

Bug: 23561678
Change-Id: I66226777671456a8919503dae4e92164ee10dd6d