Avoid RingBuffer creation on each VelocityTracker#add call

The code used to create a new RingBuffer each time a movement was added
to VelocityTracker. This is expensive, and it showed up as a regression
in our benchmark tests.

With this change, we create a new RingBuffer for a pointerId only if
there is not one already.

Bug: 267211645
Bug: 271935895
Test: atest libinput_tests, benchmark test regression fixed

Change-Id: I47c6dda5741459a1bd84f3f5f46f932c26fda523