Get rid of instance null checks

[Arikaton]

In AppsFlyer.cs script there is a lot instance null checks, which makes code readability and support worse. I suggest to convert instance filed to property and add AppsFlyerDummy instance for editor. Also move initSDK to IAppsFlyerNativeBridge interface makes the code more uniform

[af-do]

Thank you for your suggestion, we will look into it internally.