copy the lib folder, assets folder and pubspec.yaml fil to the flutter project

Run 'flutter pub get' to get all the dependency, 
Note that the packages newer version than the one used may cause error and null error

Go to android > build.gradle
Change the minSdkVersion to 23 
May subject to change based on the library package

defaultConfig {
    minSdkVersion 23
}

Go to android > app > src > main > AndroidManifest.xml
Add uses permission below after <manifest> and before <application>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.subcon_system_new">
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.USE_BIOMETRIC" />
    <uses-permission android:name="android.permission.SEND_SMS"/>
   <application ...

Go to android > app > src > main > kotlin > MainActivity.kt
add the line below to the file
import io.flutter.embedding.android.FlutterFragmentActivity

change the class to 
class MainActivity: FlutterFragmentActivity() {
}