Prevent entries with identical times

Currently, VelocityTracker adds all MotionEvents with actions DOWN,
MOVE, and POINTER_DOWN. But sometimes these events can have identical
timestamps, for example MOVE and the following POINTER_DOWN.

This breaks algorithms that rely on computing differences between
times of the events. If the difference is zero, and division is
performed, then very large numbers can result from this.

Bug: 129797752
Test: atest libinput_tests
Change-Id: I843fd9efbdbf786ab7aa5ed62b4e1395ae7cf335
Merged-In: I843fd9efbdbf786ab7aa5ed62b4e1395ae7cf335