How to acquire analog values into my android APP

Hello,
I am try to acquire analog voltage through my android app. I connected potentiometer A0 pin of the UDOO Quad board and wrote the arduino sketch was

String stringOne = String(analogRead(A0), DEC); // read the analog volatge
stringOne.getBytes(buf,sizeof(buf)); convert into the byte array

if (adk.isReady()){
adk.write(sizeof(buf),buf);//sent the usb serial.

and the android APP Code is:

public void blinkLED(View v) {
TextView txt = (TextView) findViewById(R.id.textView);
if (buttonLED.isChecked()) {
buttonLED.setBackgroundColor(Color.RED);
txt.setText(mAdkManager.readSerial());
}
else {
buttonLED.setBackgroundColor(Color.GREEN);
txt.setText("Not Connected");
}
}