Back press handling after fold/unfold

[CherryPerry]

Looks like there is slightly more to this – now a OnBackPressedCallback from a custom BackPressHandler plugin doesn't get invoked after onStop

Easiest way to repro this is in "Blocker" sample in sandbox, with same steps as before

Originally posted by @steelahhh in #102 (comment)

[CherryPerry]

This happens because of lifecycle callbacks order execution:

1. The parent node is moved to STARTED and invokes its callbacks.
2. ChildNodeLifecycleManager is the first to handle the event.
3. ChildNodeLifecycleManager moves a child to STARTED.
4. The child executes its own callbacks (order does not matter here).
5. The child registers back press callback via OnBackPressedDispatcher.LifecycleOnBackPressedCancellable.onStateChanged.
6. The parent proceeds to the next lifecycle observer which is OnBackPressedDispatcher.LifecycleOnBackPressedCancellable.onStateChanged.

After unfolding the order is different than before: child registers its callback before parent.