add NotificationService class with stubs, and test file

src/services/NotificationService.ts

@@ -0,0 +1,17 @@
+class NotificationService {
+  public startListeningForNotification() {
+
+  }
+
+  public stopListeningForNotification() {
+
+  }
+
+  public stopListeningForAllNotifications() {
+
+  }
+
+  public startReloadingNotificationTokens() {
+
+  }
+}