Move TransitionState into its own file (1/2)

This CL is a pure move of the TransitionState class into its own file.
In a follow-up CL, this class will be split into a ContentState class
that will be shared with overlay states.

Bug: 353679003
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container

Change-Id: Iab5c90f03967fba744bf91e08a662c5ffa7a4ec1