[flexiglass] Fixes infinite back nav between QS and bouncer.

Introduces a small framework for keeping track of the minimal navigation
back stack that can be used by various scenes to know which scene to
return to from a back user-action. The downside of this approach is the
need for the centralized navigationDistanceFromStart mapping.

Bug: 336335477
Test: added integration test for back navigation
Test: manually verified that QS -> edit -> bouncer -> back -> back
returns to the QS scene and then returns to the correct scene from
before the QS scene
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: Ic9dca2762e4bc1110a661b25bba60c1d84cf7eb3