Add support for movable elements

This CL adds a new MovableElement API on SceneScope to create shared
elements whose content is moved instead of being duplicated between
scenes. This allows to keep any kind of internal state during
transitions.

See b/291053742#comment5 for details and videos.

Bug: 291053742
Test: atest MovableElementTest
Change-Id: Ib62baf6764d389f18d352670c1699bbf23b0506a