[UIScene] Consistent lifecycle behavior for unattached FlutterEngine #191907

Use case

In the current state, for single scene, if a FlutterEngine is never attached to the view hierarchy.

(1) If the engine is created before scene connection notification, then it will receive all scene lifecycle events (including sceneWillConnect).
(2) If the engine is created after the notification, then it will not receive any scene lifecycle events after engine creation

This is because we add the engine to the list during notification, so if the engine misses this opportunity, then it's not tracked anymore.

Note: scene connection notification happens after sceneWillConnect delegate call, but they happen in the same run-loop.

Proposal

A more consistent behavior is, for single scenes, the engine should receive all the lifecycle events after its been created.