Fix: Don't report two app open events at initial launch

Summary

UIApplicationWillEnterForegroundNotification fires both at initial launch and when returning from the background. Because we were using a different notification (UIApplicationDidFinishLaunchingNotification) to detect if the app open was from the background, we effectively were firing the event twice. Instead, rely on UIApplicationState to determine if we were opened from the background.

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amplitude / Amplitude-iOS

Fix: Don't report two app open events at initial launch #494

Summary

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