[flexiglass] Merges BouncerSceneLayout helper and composable

No real need to have a split anymore since the Compose project has been
an integral part of the System UI build for over a year now.

Fix: 314813960
Test: manually verified with a foldable - I can see the different
bouncer layouts
Test: unit test still passes
Flag: com.android.systemui.scene_container

Change-Id: I0d4a1c21928e48d7bec8fc0d77d1d72acfe48d0a