TB 40023: [android] Stop PrivateNotificationService

Originally, android-components#40023.

mobile/android/android-components/components/feature/privatemode/src/main/java/mozilla/components/feature/privatemode/notification/PrivateNotificationFeature.kt

@@ -44,6 +44,7 @@ class PrivateNotificationFeature<T : AbstractPrivateNotificationService>(
     }
 
     override fun stop() {
+        applicationContext.stopService(Intent(applicationContext, notificationServiceClass.java))
         scope?.cancel()
     }
 }