Compose all scenes with an ongoing transition

This CL considers all running transitions when sizing and placing
elements. For each element, we now look at the last transition for which
the element is present in fromScene and/or toScene. This allows multiple
independent transitions to run in parallel.

See b/290930950#comment5 for more information about interruptions.

Bug: 290930950
Test: atest SceneTransitionLayoutTest
Test: atest ElementTest
Flag: N/A

Change-Id: Iabbb4a1d689906baf977a1aa3065d654baacd131