1.按照官方文档生成签名文件，修改build文件和manifest文件：
https://flutterchina.club/android-release/
2.注意AndroidX的适配问题，详见：
https://flutter.dev/docs/development/packages-and-plugins/androidx-compatibility#recommended-use-android-studio-to-migrate-your-app
3.release模式混淆压缩注意事项：
在 "proguard-rules.pro" 文件中，我们就要加入 -dontwarn 语句对第三方库进行报错屏蔽，否则proguard的代码混淆无法进行

#Flutter Wrapper
-dontwarn io.flutter.**
-keep class io.flutter.app.** { *; }
-keep class io.flutter.plugin.**  { *; }
-keep class io.flutter.util.**  { *; }
-keep class io.flutter.view.**  { *; }
-keep class io.flutter.**  { *; }
-keep class io.flutter.plugins.**  { *; }