Support iOS 13's new lifecycle paradigm

[mikehaney24]

iOS 13 introduces a new lifecycle paradigm with iOS 13.

According to @DavidPhillipOster, a full iOS 13 app will just get the UIScene lifecycle notifications, running on either iPhone or iPad. Before iOS 13, it will just get the UIApplication lifecycle notifications, but if it has a SceneDelegate, but does not have the dictionary in its Info.plist it will get both sets of notifications on iOS 13.

In GDT, getting multiple background/foregrounding calls is ok, so I'm solving it with #3520

Other resources:
https://stackoverflow.com/a/56508769/361918

Reach out to me (haneym@) for additional context and example fixes if needed.

[maksymmalyhin]

TODO:

• adopting of background/foreground calls for auth should be pretty straightforward. Looks like no changes needed at all (see the comment for some additional details)
• auth will have to adopt new way of handling open URL actions using UISceneDelegate which implies implementation of UISceneDelegate swizzling. Handle UISceneDelegate changes in Auth #4380
• FIRAuthDefaultUIDelegate will have to be updated to avoid using deprecated UIApplication. keyWindow property. UIApplication.connectedScenes property can be used as the alternative entry point Use UIApplication.connectedScenes on iOS 13 #4066