ExemploAdmob_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="br.com.luizmarcus.exemploadmob.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/app_name" />

    <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:adSize="SMART_BANNER"
        ads:adUnitId="ca-app-pub-5134668402335816/9169755089"
        android:layout_alignParentBottom="true"/>
</RelativeLayout>