This change adds new event that gets dispatched from native stack when screen transitioning is finished.
The new event is used by react-navigation to update the library internal state that the triggered action has been finished. Previously we were relying solely on onAppear and onDisappear events, however those does not get triggered when new iOS 13 modals are used or when transparent modals are displayed. This is because with transparent modals the view below is still visible. We therefore needed another way of notifying the library that screen transition have finished despite the fact that disappear event couldn't be triggered.
As a part of this change I also refactored invalid ref cycle-break code on iOS which was ought to remove reference cycle between view and view controller. This code have been moved to viewWillDisappear callback.
Also on Android part small refactoring has been done and we removed the necessity to keep mActiveScreens array which was occasionally getting out of sync with the list of active fragments.
The previous algorithm was buggy and did not handle the case when multiple VCs are being dismissed. Unfortunately I couldn't find a reliable way that'd allow for reshuffling modally presented VCs (inserting or deleting VCs not from the top) and so I added a special check that'd throw in the case someone attemted to do this.
On iOS by default modals show up in full-screen mode. This behavior can be customized and instead of mouting new screens in top level window they can be mounted under a given UINavController. We used to be relying on that behavior (see "CurrentContext" presentation mode). This, apparently wasn't matching the default functionality of the OS. This change is adding it as a default and keeping the old way under newly exposed presentation modes: containedModal and containedTransparentModal
Thanks to this change, iOS 13 modals can work properly.
