Integrating Xiaomi SDK with a Unity game is simply building a Plugin for Android.
Please view the following document for your reference:
1. Modify Android project and Unity Project so as to make sure that package names and versions are the same.
2. Copy Unity library to Android Project, compile Android Project and drop jar files into Unity Project \Assets\Plugins \Android\bin
3. Copy other resources under Android project into Unity project\Assets\Plugins\Android.
4. Manually compress the files using Unity File->Build Settings…