Reland "Make VelocityTracker 1D" (--try 2)

This fixes a bug where stale velocities were not cleaned up during calls
to get updated velocities.
86f72a38

Topic for the revert which is being rolled-forward: https://googleplex-android-review.git.corp.google.com/q/topic:revert-19762050-generic_vt-XWGGUQUHYR

Bug: 32830165
Test: VelocityTracker_test unaffected (atest libinput_tests)

Change-Id: Ib706e76b9502e262eabfd221bd741e5faaee1ed5