[flexiglass] Foundation for placeholder bouncer scene.

Foundational parts of the bouncer 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: 280877228
Test: unit tests included. Manually tested with the entire relation
chain in the Compose Gallery testdbed app.

Change-Id: I54449898006489fd02d1f318646d4e5317256077