fail to pop up When Use Flutter_boost

[nicorobine]

With Flutter_boost, when registering the plug-in, [UIApplication sharedApplication].delegate. delegate.windows may also be modified by function："- (BOOL)application:(UIApplication *)application DidFinishLaunchingWithOptions: (NSDictionary *) launchOptions"， The obtained rootViewController may not be the actual ViewController, when register this plug-in，which will cause the preview screen to fail to pop up

[JulianBissekkou]

This PR also fixes some issues when you have native navigation in your flutter app. In our case we push a special splash screen before launching our flutter view controller.

We had to add a small workaround to make sure that he plugin does not hold a reference to our old SplashViewController.

[JulianBissekkou]

@crazecoder