NativePlugin.cs

public class Callback : AndroidJavaProxy, IDisposable
{
	private Action action;
	private Disposable disposable;

	public Callback(Action a) : base("any.java.CallbackInterface")
	{
		action = a;
	}

	// implemented method
	public void invoke()
	{
		disposable = Observable.ReturnUnit()
			.ObserveOnMainThread()
			.Subscribe(_ =>
			{
				action?.Invoke();
			});

	}

	public void Dispose()
	{
		disposable?.Dispose();
	}
}

public class NativePlugin : MonoBehaviour
{
	private CompositeDisposable compositeDisposable = new CompositeDisposable();
	public void Call()
	{
		var callback = new Callback(() => Debug.Log("call"));
		javaObj.Call("call" callback);
		compositeDisposable.Add(callback);
	}

	void OnDestroy()
	{
		compositeDisposable.Clear();
	}
}