Different previous velocity calculation

Instead of using the instantaneous velocity as the value for the
v[i-1], use the velocity equivalent of the energy accumulated so
far. This should act as a kind of a low-pass filter, and results
in lower velocity values than the current version of the algorithm.

Bug: 35412046
Test: native and CTS
1) /data/nativetest64/libinput_tests/VelocityTracker_test
This test currently fails, and we may need to modify the
expected velocity values to get it to pass with this version
of the algorithm
2) bit CtsViewTestCases:.VelocityTrackerTest
This test passes

Change-Id: I358cfd8161ec3d423a4f3126c9bcec6de1020490