Add CLASSIFICATION_TWO_FINGER_SCROLL for touchpad scrolling

This will be used to denote the single-finger touches that
TouchInputMapper creates to simulate scrolling when the user makes
two-finger swipes on the touchpad.

Bug: 246758376
Test: check new value is sent to a test app when scrolling on a touchpad
Change-Id: Id93cba764522e36d850f7013ab8a117f64716fac