Use std::vector instead of VLA for addMovement

Currently, VLA is used to send data to 'addMovement' in VelocityTracker.
Use std::vector instead. This also helps assert against the number of
pointers present in the idBits.

Bug: 167946721
Test: atest libinput_tests
Change-Id: Ie8aabafe9a407a2b07d3edef4d1fa312510ccdba