Report motion offsets for touchpad swipes

Adds two new axes, AXIS_GESTURE_X_OFFSET and AXIS_GESTURE_Y_OFFSET,
which report the movement of swipe gestures on the touchpad as a
fraction of the touchpad's size.

Bug: 246758376
Test: check axis values come through in a test app
Change-Id: I313410053a8db13273bd05a33d3a6a1f75081dae