Improve scroll responsiveness.

1) mUsePagingTouchSlop set to false.
   Applist scroll now scrolls with the same
   thresholds used by homescreen scrolling.

2) Early start of scroll as soon as we
   detect that it's okay to scroll.
   Eliminates waiting for next vsync
   and starts scrolling in the current cycle.
   instead of the next.

Above changes help in reducing first touch to
first displacement latencies in scroll usecases.
Improves scroll responsiveness.

Change-Id: Ib45a361b9b6275e64d0a5b5603aa15fdd386a5f8