Merge changes from topics "stl-movable-refactor", "stl-picker-rename",...

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)