How to stop runnable when the app goes to background?

Question

I am trying to establish a runnable which can load ads by every 5 sec interval (of course 5 sec is too fast, it's just for testing purpose)

Here is my code:

package com.admobsdk_dfp_handler;

import com.google.ads.*;
import com.google.ads.doubleclick.*;

import android.os.Bundle;
import android.os.Handler;
import android.app.Activity;
import android.view.Menu;
import android.widget.RelativeLayout;

public class AdMobSDK_DFP_Handler extends Activity {
    private DfpAdView adView;
    private Handler handler = new Handler();
    private Runnable runnable = new Runnable() {

        public void run() {
        adView.loadAd(new AdRequest());
        handler.postDelayed(this, 5000);
    }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_ad_mob_sdk__dfp__handler);

        adView = new DfpAdView(
                this,
                AdSize.BANNER,
                AD_UNIT_ID);

        RelativeLayout layout = (RelativeLayout) findViewById(R.id.mainLayout);

        layout.addView(adView);

        adView.loadAd(new AdRequest());

        handler.postDelayed(runnable, 5000);


    };

    @Override
    protected void onDestroy() {
        handler.removeCallbacks(runnable);
        super.onDestroy();
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_ad_mob_sdk__dfp__handler,
                menu);
        return true;
    }

}

If I press home button to hide the app to the background, the runnable keeps loading ads by 5 sec interval.

Is there any method to stop runnable when an app is hidden to the background? Many thanks.