[flexiglass] Foundation for placeholder lock screen scene.

Foundational parts of the lock screen 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: 280879610
Test: unit tests included. Manually tested with the entire relation
chain in the Compose Gallery testdbed app.

Change-Id: I90b025f57f993c02368e73a06fba6e4ddc02b9bc