[flexiglass] Introduces a previousScene flow.

This is needed for bugs like the attached one and potentially other bugs
where logic needs to know not only the current scene but also the
previous scene but don't need to monitor every scene-to-scene transition
state.

Bug: 330695017
Test: added unit tests at the data and domain layers
Test: manually verified the fix for the attached bug with the next CL in
the chain
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: Ifb07f26f47a210d46b17fdbcc665dbc6f8c272b5