Expose glanceable hub transition state

This CL exposes the full transition state of the glanceable hub's
SceneTransitionLayout so that other pieces of SysUI can use it for
transitions.

Bug: 315203819
Bug: 315490861
Fixed: 315203819
Test: atest CommunalRepositoryImplTest
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Change-Id: Ic496a40657ec6cd7e7c548d703de2c8be01d2788