[minor] FirebaseAnalytics.SetCurrentScreen should have screen class be optional

[Thaina]

FirebaseAnalytics.SetCurrentScreen has 2 argument. But second argument should be optional for convenient, no need to FirebaseAnalytics.SetCurrentScreen("name",null)

[google-oss-bot]

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[paulinon]

Hi @Thaina The screenClass is actually needed for the SetCurrentScreen method when a user intends to use another screen class that isn't the default Activity or UIViewController. I'll still treat this as a feature request, so I created an internal FR for this. With that being said, I'll keep this open. Thanks!