混合开发如何设置第一个页面为 Flutter 页面

1.用 flutter create 命令新建一个项目
2.加入boost

问题
1. 如何让第一个页面为 FlutterBoostActivity
2. 物理退后按钮失效，点击之后直接退出应用，我发现这个问题，然后猜测应该要让 MainActivity 为 FlutterBoostActivity

谢谢

[flutter_boost-e19bf36cb451e2790edb65d353a643dbd472f8fa.zip](https://github.com/alibaba/flutter_boost/files/8040323/flutter_boost-e19bf36cb451e2790edb65d353a643dbd472f8fa.zip)

Start with the attached flutter_boost reproduction archive and trace how the initial native Activity is configured, focusing on FlutterBoostActivity and MainActivity. Reproduce the startup and physical back-button behavior; done means the first page opens as a FlutterBoostActivity and the back button no longer exits the application unexpectedly.

Written by the indexing model from the issue text.