I had the similar task to configure ESP8266 without Internet access, directly connecting via phone with app. I used Network library, regular socket, and AsyncStreamText module at Android side. On ESP8266 there's WiFiServer and WiFiClient. It works as clear text protocol like Telnet. Later I have added web interface on ESP8266 using other port number, but found chip performance unsatisfactory for it - so I still use the first method of configuration.