Tạo đồng hồ đếm ngược count down timer trên android 2.3 - android N

import android.os.CountDownTimer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    TextView txt;
    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        txt = (TextView) findViewById(R.id.textViewSo);

        new CountDownTimer(10000,1000){
            @Override            public void onTick(long millisUntilFinished) {
                txt.setText("Con lai: "+ millisUntilFinished/1000+" s");

            }

            @Override            public void onFinish() {
                txt.setText("oop Time Out!");

            }
        }.start();
    }