Per-pointer processing in VelocityTracker

We would like to skip resampled data in VelocityTracker when the
velocity is being computed. To make this happen, we need to first
process the data per-pointer. This will then allow us to skip individual
data points.

To minimize the impact to older VT strategies that are largely untested,
the main change here is to make 'mMovements' and 'mIndex' become
per-pointer, rather than converting to a vector.

Also, use std::array instead of [] because [] cannot be used inside a
std::map easily.

There should be no functional change in this CL. The actual skipping of
resampled values will be done in a separate CL.

Bug: 167946721
Test: m libinput_tests && $ANDROID_HOST_OUT/nativetest64/libinput_tests/libinput_tests

Change-Id: I1c3c845bca0d4bb7d2c3973bfe84462139ac36f3