Arduino to Android serial communication issue
Right after posting this question, I've found the issue: I was looking at the wrong website for the library. The answer was posted on their wiki already.

From the library's wiki:


  Some Arduinos use the DTR line to determine serial channel readiness. In your Android code, call setDTR(true);


Library's wiki: https://github.com/mik3y/usb-serial-for-android/wiki/Troubleshooting#i-am-using-an-arduino-uno-sparkfun-pro-micro-or-other-arduino-and-if-serial-doesnt-work