Shade back action handling for scenes

Extracts the shade's contract with the back action interactor from
ShadeViewController into its own interface. Adds a new implementation
of that interface backed by the scene container.

Test: added new test
Test: manual
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Fixes: 323579909

Change-Id: I27fc9c69ebd50641e02a5eb3eb9c1d958e7ef3e0