code: public void shutdown() {
      if (this == singleton) {
        throw new UnsupportedOperationException("Default singleton instance cannot be shutdown.");
      }
      if (shutdown) {
        return;
      }
      application.unregisterActivityLifecycleCallbacks(activityLifecycleCallback);
      // Only supplied by us for testing, so it's ok to shut it down. If we were to make this public,
      // we'll have to add a check similar to that of AnalyticsNetworkExecutorService below.
      analyticsExecutor.shutdown();
      if (networkExecutor instanceof AnalyticsNetworkExecutorService) {
        networkExecutor.shutdown();
      }
      stats.shutdown();
      shutdown = true;
      synchronized (INSTANCES) {
        INSTANCES.remove(tag);
      }
    }
docs: Stops this instance from accepting further requests.