android 在<meta-data android:name="io.flutter.embedding.android.SplashScreenDrawable" android:resource="@drawable/page_loading"/>，example里默认是一张静态图片，gif图片参考https://stackoverflow.com/questions/39871405/can-i-add-gif-format-image-as-a-splash-screen，关闭的话重写掉splash方法
ios 默认应该关闭的，参考https://github.com/alibaba/flutter_boost/issues/644