fix: set minSdkVersion for Flutter Secure Storage

examples/flutter-user-management/android/app/build.gradle

@@ -44,7 +44,7 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "io.supabase.supabase_demo"
-        minSdkVersion flutter.minSdkVersion
+        minSdkVersion 18
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName