Merge changes from topics "stl-movable-refactor", "stl-picker-rename", "stl-remove-isbackground", "stl-ucs" into main
* changes:
Introduce MovableElementScenePicker
Remove ElementKey.isBackground (1/2)
Rename SharedElementScenePicker to ElementScenePicker (1/2)
Make (Movable)ElementScope extend ElementBoxScope
Work around b/317972419
Make the Element map a normal Map
Extract movable contents outside of the Element class
Rename Element.TargetValues to Element.SceneState
Extract shared values outside of the Element class
Replace AnimatedState.valueOrNull by unsafeCompositionState (1/2)
Refactor animate(Scene|Element)FooAsState (1/2)
Make animated values normal State objects
Compose movable elements in the scene picked by the ScenePicker (1/2)
Split SceneScope.MovableElement with SceneSope.Element (1/2)
Move scene picker definition inside ElementKey (1/2)