Use TWO_FINGER_SWIPE classification for touchpad scroll events

This allows apps to distinguish the fake finger created for touchpad
scrolling from an actual finger.

Bug: 246758376
Test: add classification to InputDispatcher's outbound event logs and
      check the new value is used when scrolling
Change-Id: Ia90f9984e75ad6fde2d0e42628ab42eab371b7a5