Implemented structure for push Notifications

android/app/build.gradle

@@ -23,6 +23,7 @@ if (flutterVersionName == null) {
 
 apply plugin: 'com.android.application'
 apply plugin: 'kotlin-android'
+apply plugin: 'com.google.gms.google-services'
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
@@ -39,7 +40,7 @@ android {
         applicationId "eu.mhsl.marianum.mobile.client"
         // You can update the following values to match your application needs.
         // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-build-configuration.
-        minSdkVersion flutter.minSdkVersion
+        minSdkVersion 19
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName