Obviously I do not expect any real ads to appear, but some indication that I've done the procedure correctly to find out if that's all right.
First image (Preview Android Studio)
2ndimage(Virtualmachineapplication)
Asshownnothingappears,soIhavenoidea.
ActivityXML
<com.google.android.gms.ads.AdViewandroid:id="@+id/adView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
ads:adSize="BANNER"
ads:adUnitId="@string/banner_ad_unit"/>