Currently FlutterViewController has a private method called launchEngine that's called from viewWillAppear. That registers internal plugins, starts the engine, and sends the initial settings. There's no public way to trigger that flow, which means that if a developer wants to create a FlutterViewController that's hidden, but start running it, they can't; if they start running the engine directly using engine methods, then it won't get the internal plugins or settings.

We should change the way that non-engine code is triggered, or if that's not viable, add public API to trigger that flow.