Connect arduino to Android with OTG cable. Arduino serial controller helps you to communicate with arduino.
This project uses usb-serial-for-android driver. Get details from below 2 repository. https://github.com/mik3y/usb-serial-for-android https://github.com/andreasb242/usb-serial-for-android
To compile with USB-Serial driver, you must import below path to your project as external source link.
Arduino-Serial-Controller/Android/usbSerialForAndroid/src/main/java