title: elevation not working in LinearLayout
body: <p>bg_book_promotion is the picture which needs to have a shadow
  when I add the <code>android:elevation</code>, it is not working.
  which one can tell me how to correct it, it is so strange.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:background="@color/color_fff_white"
      android:orientation="vertical"&gt;
  
      &lt;LinearLayout
              android:id="@+id/ll_promotion_bg"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:layout_marginBottom="@dimen/margin9"
              android:layout_marginLeft="@dimen/margin7"
              android:layout_marginRight="@dimen/margin7"
              android:layout_marginTop="@dimen/margin14"
              android:background="@drawable/bg_book_promotion"
              android:elevation="@dimen/height2"
              android:gravity="center_vertical"
              android:orientation="vertical"
              android:paddingLeft="@dimen/padding9"&gt;
  
              &lt;TextView
                  android:id="@+id/tv_promotion_title"
                  android:layout_width="wrap_content"
                  android:layout_height="wrap_content"
                  android:textColor="@color/color_fff_white"
                  android:textSize="@dimen/textsize15"
                  android:textStyle="bold"
                  tools:text="" /&gt;
  
              &lt;TextView
                  android:id="@+id/tv_promotion_desc"
                  android:layout_width="wrap_content"
                  android:layout_height="wrap_content"
                  android:layout_gravity="center_vertical"
                  android:textColor="@color/color_fff_white"
                  android:textSize="@dimen/textsize12"
                  tools:text="" /&gt;
          &lt;/LinearLayout&gt;
   &lt;/LinearLayout&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p><a href="https://i.stack.imgur.com/iV8Ew.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iV8Ew.png" alt="enter image description here"></a></p>