code: public static StitchAppClient initializeAppClient(
        @Nonnull final String clientAppId
    ) {
      return initializeAppClient(clientAppId, new StitchAppClientConfiguration.Builder().build());
    }
docs: Initializes an app client for Stitch to use when using {@link Stitch#getAppClient(String)}}.
  Can only be called once per client app id.
  
  @param clientAppId the client app id to initialize an app client for.
  @return the app client that was just initialized.