Use vector in VelocityTracker_test

Using vector prevents the need for passing around the 'count' variable,
and also reduces the number of lines of code.

Bug: none
Test: atest -a libinput_tests inputflinger_tests
Change-Id: I18b28a3a6b183b26316299d477892d15ad9c67a2
Merged-In: I18b28a3a6b183b26316299d477892d15ad9c67a2