Handle DOWN/MOVE/UP gestures with fling.

Bug: 5265814

When MOVE events are batched, a ListView may not receive only
one MOVE event followed immediately by an UP in the case of
a quick fling.  Unfortunately, the old code relied on there
being at least one additional MOVE event following the first
in order to really scroll or fling.

This change extracts common code from onTouchEvent to
handle scrolls as soon as they are started if there is additional
remaining motion beyond the initial touch slop (taken care of by
mMotionCorrection).

Change-Id: I8c8f03f580bfba96aa69cf111d245ba7fd3b4c2a