adds firebase connection and notifications

- notification handler pushes background and
  foreground notifications to the phone directly
- firebase connection pushes remote notifications
  in background and foreground
- firebase unique token currently logged only, should
  be send directly to server via rest

app/build.gradle

@@ -38,6 +38,7 @@ dependencies {
 
     implementation 'com.google.android.gms:play-services-mlkit-barcode-scanning:16.1.4'
     implementation 'com.android.volley:volley:1.2.0'
+    implementation 'com.google.firebase:firebase-messaging-ktx:21.0.1'
     def camerax_version = "1.0.2"
     implementation 'androidx.core:core-ktx:1.3.2'
     implementation 'androidx.appcompat:appcompat:1.2.0'
@@ -53,4 +54,6 @@ dependencies {
     testImplementation 'junit:junit:4.+'
     androidTestImplementation 'androidx.test.ext:junit:1.1.2'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
-}
+}
+
+apply plugin: 'com.google.gms.google-services'  // Google Play services Gradle plugin