title: LinearLayout fails when set to be parent of ScrollView
body: <p>I want to make that a LinearLayout displays an ad in the bottom of the screen. It's a banner ad.
  I have tried many combinations and when I put the LinearLayout above the Scrollview in the layout.xml hierarchy, I get an error of the type "ScrollView can host only one direct child" and the app crashes.</p>
  
  <p>I am able to make the app to work by putting the ScrollView above the LinearLayout of the ad, but in this case the ad appears below all the buttons while it should appear in the hierarchy over the scrolling buttons, at the bottom of the screen.</p>
  
  <p>How can I manage to make that the banner appears at the bottom of the screen with the scrolling buttons above?</p>
  
  <p>This is an excerpt of my main.xml file:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;!-- This file is /res/layout/main.xml   Era LinearLayout--&gt;
  
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:orientation="vertical"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"&gt;
  
      &lt;!--  
          &lt;com.facebook.ads.AdView
              android:id="@id/banner"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:layout_alignParentBottom="true"
               /&gt;
      --&gt;
  
  
        &lt;ScrollView
            android:id="@+id/scroll"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"&gt;
  
         &lt;LinearLayout
             android:id="@+id/banner_container"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:layout_alignParentBottom="true"
             android:orientation="vertical"
             app:layout_constraintBottom_toBottomOf="parent"&gt;
  
            &lt;Button
                android:id="@+id/holaPlayerBtn"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:background="@drawable/custom_button_yellow"
                android:text="@string/hola"
                android:textColor="#000000"
                android:textStyle="bold" /&gt;
  
            &lt;Button
                android:id="@+id/adiosPlayerBtn"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:background="@drawable/custom_button_yellow"
                android:text="@string/adios"
                android:textColor="#000000"
                android:textStyle="bold" /&gt;
  
  
            ... many other buttons here ...
  
            &lt;Button
                android:id="@+id/hastaprontoPlayerBtn"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:background="@drawable/custom_button_yellow"
                android:text="@string/hasta_pronto"
                android:textColor="#000000"
                android:textStyle="bold" /&gt;
  
         &lt;/LinearLayout&gt;
     &lt;/ScrollView&gt;
  
  &lt;/RelativeLayout&gt;
  </code></pre>