Compose movable elements in the scene picked by the ScenePicker (1/2)

This CL simplifies the logic of MovableElement so that we always compose
a MovableElement in the scene returned by the associated ScenePicker.

Doing this can sometimes break the animation of a movable element if it
was never compose in a scene before the transition, as explained in
b/317026105#comment1. I will add more utilities to help work around this
limitation in a follow-up CL.

Bug: 317026105
Test: PlatformComposeSceneTransitionLayoutTests
Flag: N/A
Change-Id: Icf4ab837b51cc1321ba7e1bd502bbebebd3a2eac