code: public WalletAppKit setUserAgent(String userAgent, String version) {
          this.userAgent = checkNotNull(userAgent);
          this.version = checkNotNull(version);
          return this;
      }
docs: Sets the string that will appear in the subver field of the version message.
  @param userAgent A short string that should be the name of your app, e.g. "My Wallet"
  @param version A short string that contains the version number, e.g. "1.0-BETA"