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