Flutter的默认启动屏是个白色的背景，
android12之后启动方式改成图标启动
flutter_native_splash插件放在开发依赖，这样就不用打包到发布版本中，减少 APP 尺寸。

pubspec.yaml

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_native_splash: ^2.4.3
  
# 启动图适配 android 11 及以下, 12 以上，IOS
flutter_native_splash:
  web: false
  color_android: "#ffffff"
  background_image_android: "images/icon.png"
  background_image_ios: "images/icon.png"
  # image_ios: "assets/launcher/android.png"
  android_12:
    image: "images/icon.png"
    # icon_background_color: "#324ea1"

项目根目录执行

生成启动屏：dart run flutter_native_splash:create 
或删除启动屏：dart run flutter_native_splash:remove 

启动后看到效果，自己定义的icon.png已经生效了