[fix] 适配 flutter_webview_plugin 插件

[foxsofter]

目前引入这个插件会出现两个FlutterEngine在跑，根本原因是 flutter_webview_plugin 中的下面这段代码

+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
    channel = [FlutterMethodChannel
               methodChannelWithName:CHANNEL_NAME
               binaryMessenger:[registrar messenger]];

    UIViewController *viewController = [UIApplication sharedApplication].delegate.window.rootViewController;
    FlutterWebviewPlugin* instance = [[FlutterWebviewPlugin alloc] initWithViewController:viewController];
    
    [registrar addMethodCallDelegate:instance channel:channel];
}

直接获取 rootViewController 作为默认的 FlutterViewController，太过于简单粗暴，正确的方式应该是从 FlutterEngine 获取

[foxsofter]

无法适配，需要 flutter_webview_plugin 本身作修改

[foxsofter]

这个插件看似已经没人维护了，不管了