Tweak VelocityTracker.

Bug: 5265529

Calculate the velocity using the most recent touch sample as the
point of reference instead of the oldest.  This change more heavily
weights recent touch samples and reduces the sample time window
used for calculation.  This significantly improves the accuracy
of fling gesture detection.

Change-Id: Ib1940933e786e5f6a731552a99bcd9400741d55f