Move TransitionKey to Transition

This CL simply moves the TransitionKey associated to a transition inside
Transition itself, rather than binding it in STLState.startTransition.

Bug: 290930950
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: I371c8b93ab6c216e7f89b9c06e41361287cf2094