[flexiglass] Adds pointerCount and fromEdge to the UserAction model.

This should help us support cases like:
- When Gone, swiping down from the top edge with two fingers --> quick
  settings
- When on Lockscreen, swiping down from the top edge --> quick settings

Work still needs to happen in SceneTransitionLayout to actually support
detecting how many input pointers are down and which edge (if any) the
swipe started from.

Bug: 291055080
Test: built and launched with Flexiglass, can still navigate from Gone
to Shade and from Lockscreen to Shade. Two finger or edge swiping
doesn't work yet but it should work once the SceneTransitionLayout parts
are added.

Change-Id: I333a8c540051adffcf04e3b15cecc062bb2ebdfe