add NotificationService method and test case stub

src/services/NotificationService.ts

@@ -250,4 +250,12 @@ export class NotificationService {
     }
     return this.deviceIdsToDeliverTo[tuple.toString()].has(deviceId);
   }
+
+  /**
+   * Return all scheduled notification for the given device ID.
+   * @param deviceId
+   */
+  public async getAllScheduledNotificationsForDevice(deviceId: string): Promise<ScheduledNotificationData[]> {
+    return [];
+  }
 }