Activity_1.java

    String s = "";
    int counter = 1;

    public void addLine(View view) {
        TextView tv = findViewById(R.id.test_text);
        s = s + "Line " + counter + "<br>";
        counter++;

        tv.setText(Html.fromHtml(s, Html.FROM_HTML_MODE_LEGACY));
        int scrollAmount = tv.getLayout().getLineTop(tv.getLineCount()) - tv.getHeight();
        if (scrollAmount > 0)
            tv.scrollTo(0, scrollAmount);
        else
            tv.scrollTo(0, 0);
    }