Use movement of the motion events on the screen instead of on the

trackpad since they are proportional

This simplifies the logic we have to track the movements in the code.
Also a slight fix to not cancel back gesture on pointer down from trackpad

Bug: 255697805
Test: 3 finger swipe to go back with the flag turned on
Change-Id: Ie85ad1d61f846ece8e515ddd456bd1729a45354e