added persistence using shared preferences

android/app/src/main/kotlin/com/example/maps_bookmarks/MainActivity.kt

@@ -5,6 +5,7 @@ import android.os.Bundle
 import io.flutter.embedding.android.FlutterActivity
 import io.flutter.embedding.engine.FlutterEngine
 import io.flutter.plugin.common.MethodChannel
+import io.flutter.plugins.sharedpreferences.SharedPreferencesPlugin
 
 class MainActivity: FlutterActivity() {
     private var sharedText: String? = null
@@ -31,6 +32,8 @@ class MainActivity: FlutterActivity() {
     }
 
     override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
+        super.configureFlutterEngine(flutterEngine)
+        flutterEngine.plugins.add(SharedPreferencesPlugin())
         MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL)
             .setMethodCallHandler { call, result ->
                 if (call.method == "getSharedText") {