Use ContentState.Transition in Element.kt

This CL is a pure refactoring that replaces most usages of
TransitionState.Transition in Element.kt by ContentState.Transition.
This will allow Modifier.element() to work out of the box with overlay
transitions that are going to be added in a follow-up CL.

Bug: 353679003
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: I280e14aa09e5448e711300741efdf83b139a1614