You can simple set :
<com.google.ads.AdView android:id="@+id/ad"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
ads:adSize="BANNER"
android:gravity="bottom"
ads:adUnitId="xxxxxxxxxxx"
ads:testDevices="TEST_EMULATOR"
ads:loadAdOnCreate="true"/>
or You can set in LinearLayout:
<LinearLayout android:id="@+id/ad"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom"
/>