[flexiglass] Foundation for placeholder shade scene.

Foundational parts of the shade scene including application state
(which is still not hooked up to real sources of truth), business logic,
and a view-model that's only good enough for placeholder scene UI.

Bug: 280887022
Test: unit tests included. Manually tested with the entire relation
chain in the Compose Gallery testdbed app.

Change-Id: If1a64a9346343ad2242c39049d1a57ba7c8d76a8