Explore restoring `WorkflowSavedStateRegistryAggregator` eagerly in `attachToParentRegistry` if the parent registry is already restored at that time

[zach-klippenstein]

Justification: View/Compose interop. It's possible to be provided an already-restored registry, and in that case there's no reason to wait to restore it. It's also possible that the parent Lifecycle won't move past INITIALIZED until after an attach call (LifecycleOwner -> View with Aggregator), and in that case if a ComposeView is a child of the Aggregator's View, the ComposeView will throw when it tries restoring from the registry.

If this works, then in this case we shouldn't need to register the LifecycleObserver at all.