Improve VelocityTracker Strategy Handling

The native VelocityTracker class has been made 1-dimensional to allow
support for axes beyond the historically-supported X/Y axes. This means
that a given VelocityTracker instance does not necessarily handle data
for all supported axes. As such, this CL sets up tracking strategy for
an axis only on the first occassion a data arrives for the axis.
Furthermore, to support use cases where different strategies may suit
different axes better, we have introduced per-axis default strategies.

Bug: 32830165
Test: atest libinput_tests; manual on-device fling tests
Change-Id: I3f7115fdcec78d1577e90e9a55175e8868bf2dfb