Extract ContentState out of TransitionState (1/2)

This CL is a pure extraction of TransitionState into a ContentState
class, meant to be subclassed by OverlayState in follow-up CLs.

Bug: 353679003
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: I1a0776412bf9335a91d3ced3af36ff5748f28ccd