0-activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:orientation="vertical">

  <com.google.android.gms.ads.AdView xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/adView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    ads:adUnitId="@string/admob_add_unit_id"
    ads:adSize="SMART_BANNER" />

</LinearLayout>

0-AdMobPokerExampleActivity.java

public class AdMobPokerExampleActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        AdMobPoker.track(findViewById(R.id.adView), savedInstanceState);
    }

AdMobPoker.java

import android.app.Activity;
import android.app.Application;
import android.os.Bundle;
import android.view.View;

import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;

public class AdMobPoker implements Application.ActivityLifecycleCallbacks {

    private final AdView adView;

    public static void track(View adView, Bundle savedInstanceState) {
        if (!(adView instanceof AdView)) {
            throw new IllegalStateException("The view you pass must be an AdMob view.");
        }
        AdMobPoker adMobPoker = new AdMobPoker((AdView) adView);
        Activity activity = (Activity) adView.getContext();
        activity.getApplication().registerActivityLifecycleCallbacks(adMobPoker);
        adMobPoker.onActivityCreated(activity, savedInstanceState);
    }

    private AdMobPoker(AdView adView) {
        this.adView = adView;
    }

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        AdRequest adRequest = new AdRequest.Builder()
                .addTestDevice(AdRequest.DEVICE_ID_EMULATOR)
                .build();
        adView.loadAd(adRequest);
    }

    @Override
    public void onActivityStarted(Activity activity) {
        // not used
    }

    @Override
    public void onActivityResumed(Activity activity) {
        adView.resume();
    }

    @Override
    public void onActivityPaused(Activity activity) {
        adView.pause();
    }

    @Override
    public void onActivityStopped(Activity activity) {
        // not used
    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        // not used
    }

    @Override
    public void onActivityDestroyed(Activity activity) {
        adView.destroy();
        activity.getApplication().unregisterActivityLifecycleCallbacks(this);
    }
}