Implement an integrating VelocityTracker strategy.

This algorithm better tolerates certain kinds of errors in the
touch input than the least squares strategy but it may underestimate
the velocity of accelerating movements.  This algorithm is
mainly of interest as a baseline for testing and comparison with the
least squares algorithm, which remains the default.

Bug: 6413587
Change-Id: I8ddb50084e44875e234717907e5b06d03f59788c