VelocityTracker Cleanups: Differential Axes Support Follow Up

This CL follows ag/19950271. It:
- fixes a param name, to make it consistent with the latest terminology we use (and the one we use in the .cpp file)
- add comment to the VelocityTrackr#createStrategy function about this param

Bug: 32830165
Change-Id: Idf90547c6b9629e98ee9014549085a1db9024700
Test: tests unaffected (trivial naming/comment change)