The FragmentManager should be converted over to saving its state using the SavedStateRegistry architecture component.

We should provide FragmentManager with a SavedStateRegistryController and move all of the code from the FragmentActivity's init() into the attachController() method of FragmentManager.

It currently relies on the SavedStateRegistry of the FragmentActivity which hacked together methods like markFragmentsCreated() and mFragments.saveAllState() that can likely be called directly from the FragmentManager.

The more complex piece is moving the OnContextAvailableListener() over since FragmentManager does not have the same signal for when the Context is immediately available.