Adds NavigationEdgeBackPlugin to iterate on different Back implementations.

NavigationBarEdgePanel now implements the new plugin. As part of that,
some of the UI-specific logic from EdgeBackGestureHandler has been
moved to NavigationBarEdgePanel (e.g. Dark region detector and logic
around the arrow position).

Test: Built and manually tested existing back behavior.
Fixes: 143907351
Change-Id: I0e01581d6b69e6a8a03a4338be83ed5ff8b2da65