[flexiglass] SceneContainer in System UI.

If the Jetpack Compose compile-time flag and the scene container runtime
feature flag are both true, when we inflate the root
SceneWindowRootView, we also initialize it with its required
dependencies and add a ComposeView that hosts the SceneContainer
composable function.

This effectively adds the placholder scenes developed through the
Compose gallery app to actual System UI, allowing us to proceed to the
next steps of the project where we being connecting Flexiglass to real
business logic and application state.

Note that the back navigation integration done requires the enablement
of predictive back animation for the shade as described in
go/enable-notification-shade-anim.

Bug: 283836103
Test: manually verified that the Flexiglass placholder scenes correctly
show on top of the shade, correctly hide when the window is gone, can
correctly be navigated between using the placholder up/down buttons, and
the bouncer scene correctly supports the back gesture to go back to
lockscreen.

Change-Id: I82c864e922ce77469ae7fbc5f0cef77e6947ef3f
Merged-In: I82c864e922ce77469ae7fbc5f0cef77e6947ef3f