[flexiglass] SceneDataSource.

In order to support the new TransitionKey API in SceneTransitionLayout
(STL), we must switch from using HoistedSceneTransitionLayoutState to
using MutableSceneTransitionLayoutState.

Unlike HoistedSceneTransitionLayoutState, the MutableSceneTransitionLayoutState keeps the source of truth of the
current scene within itself, forcing us to change our architecture in
order to source the state of the current scene from
MutableSceneTransitionLayoutState and not from our own repository.

This is the first CL in a chain. Here we introduce the SceneDataSource
interface which, in the next CL, will be used by our
SceneContainerRepository as the new source of truth.

Complicating matters is the fact that MutableSceneTransitionLayoutState
is a Compose-aware construct that must be instantiated (and bound to)
within an @Composable function. Therefore, we introduce both
SceneTransitionLayoutDataSource (a MutableSceneTransitionLayoutState
based implementation of SceneDataSource) and SceneDataSourceDelegator
which also implements SceneDataSource and can be provided into our
Dagger graph as a normal dependency in build-time but can receive a
SceneDataSource as a delegate, during runtime. By doing that, we allow
our code in the data layer (the repository) depend on an object provided
from the UI layer (the composable) without breaking the Clean Architecture
Dependency Rule and without adding a dependency on Jetpack Compose from
a part of the codebase that's not yet allowed to do that.

Bug: 323173116
Test: unit test included
Test: unit tests updated in following CL(s)
Test: manual verification done with following CL(s)
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Ic1aa6aafb6bc8b7d8a133e441a782d377b1ff4e4