[flexiglass] Fixes crash when opening QS from Shade

Gotta filter backScene to not ever emit the same scene from that scene's
view-model. Other scenes that use backScene already do this.

Fix: 370962149
Test: manually verified that the crash no longer occurs
Test: unit test added to reproduce the failure and it no longer
reproduces after the fix
Flag: com.android.systemui.scene_container

Change-Id: I4e643ed6116b0694af524f6532d09f84e9f0a83d