Add VelocityTracker Support for AXIS_SCROLL

AXIS_SCROLL motion events report diffs in movement between successive
events, so we have introduced a concept of cumulative-axes in this CL to
support AXIS_SCROLL.
Demo: go/rsb-fling-demo-1

Bug: 32830165
Test: atest libinput_tests; manual fling with rotating side button

Change-Id: Ic124b5c6d4d4bc30d1a45c49be4e5e8856d6c974