On each click, you are replacing the text in 'resultTextView' with what is in 'inputEditText', albeit with a linebreak on the end. You are not adding to what was there before.
I think this is what you are after:
resultTextView.setText(resultTextView.getText() + "\r\n" + inputEditText.getText());
So you set it to what it currently is, plus a new line break, plus the new line from the edittext.