findViewById - in separate class
You could also make a baseactivity for all the other activity that are going to use the adView

    public class BaseActivity extends Activity {

    private DfpAdView adView;
    RelativeLayout rl;
    //some other stuff

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.baseactivity_layout)
        rl = (RelativeLayout) findViewById(R.id.adLayout);
        adView = new DfpAdView(this, AdSize.BANNER, adId);
        rl.addView(adView);
        adView.loadAd(new AdRequest());
    }
}


And and make your MainActivity (or all the other activities that are using the adview)

public class MainActivity extends BaseActivity {
    @Override
    public void onCreate(Bundle savedInstanceState){

    }
}