Support manually adding Google Service files without using flutterfire configure

[russellwheatley]

If you try and manually add google service config files to your android and iOS project (i.e. not using flutterfire configure), it tries to call optionsFromResouce(). This behaviour needs to change to account for users who have not used the FlutterFire CLI. The condition for this line needs to change: https://github.com/firebase/flutterfire/blob/ea03c6b349ccd63317f96e37c85daf7e11f2a28a/packages/firebase_core/firebase_core_platform_interface/lib/src/method_channel/method_channel_firebase.dart#L89