Tweak velocity tracker.

Bug: 5265529

New velocity tracker algorithm prefers more recent data points.
We can observe this effect in the test by noticing that after
moving at constant velocity for 100ms and another 10ms later
the velocity reported by the VelocityTracker is unchanged.

Change-Id: I0a71e62ade0937d9de7702925d701e8e95dc16e6