Ignore resampled values when computing velocity

VelocityTracker should only operate on the actual data produced by the
touchscreen. It should not use the fake, synthesized data during the
computation.
Using such data can cause unwanted artifacts such as backwards fling,
etc.

Bug: 167946721
Test: m libinput_tests && $ANDROID_HOST_OUT/nativetest64/libinput_tests/libinput_tests
Change-Id: I1ffdc1d38b0b335419f20c7bdd0d4473942bbf57