[flexiglass] Foundation for placeholder QS scene.

Foundational parts of the QS 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: 280887232
Test: unit tests included. Manually tested with the entire relation
chain in the Compose Gallery testdbed app.

Change-Id: Ia719fce3a3e089a7aaf677ed321b853cd0acd418