title: Adding expandableListView to NavigationView
body: <p>I have a menu defined in xml:</p>
  
  <p></p>
  
  <pre><code>&lt;group android:checkableBehavior="single" android:id="@+id/group"&gt;
      &lt;item
          android:id="@+id/grades"
          android:icon="@drawable/ic_font_download_black_48dp"
          android:checked="false"
          android:title="Grades" &gt;
          &lt;menu&gt;
              &lt;item
                  android:id="@+id/mp1"
                  android:icon="@drawable/ic_looks_one_black_24dp"
                  android:checked="false"
                  android:title="MP1" /&gt;
              &lt;item
                  android:id="@+id/mp2"
                  android:icon="@drawable/ic_looks_two_black_24dp"
                  android:checked="false"
                  android:title="MP2" /&gt;
              &lt;item
                  android:id="@+id/mp3"
                  android:icon="@drawable/ic_font_download_black_48dp"
                  android:checked="false"
                  android:title="MP3" /&gt;
              &lt;item
                  android:id="@+id/mp4"
                  android:icon="@drawable/ic_font_download_black_48dp"
                  android:checked="false"
                  android:title="MP4" /&gt;
          &lt;/menu&gt;
      &lt;/item&gt;
      &lt;item
          android:id="@+id/schedule"
          android:icon="@drawable/ic_event_black_48dp"
          android:checked="false"
          android:title="Schedule" /&gt;
      &lt;item
          android:id="@+id/attendance"
          android:icon="@drawable/ic_assignment_ind_black_48dp"
          android:checked="false"
          android:title="Attendance" /&gt;
      &lt;item
          android:id="@+id/assignments"
          android:icon="@drawable/ic_assignment_black_48dp"
          android:checked="false"
          android:title="Assignments" /&gt;
      &lt;item
          android:id="@+id/studentInfo"
          android:icon="@drawable/ic_account_circle_black_48dp"
          android:checked="false"
          android:title="Student Details" /&gt;
  &lt;/group&gt;
  &lt;group android:id="@+id/group2" android:checkableBehavior="single"&gt;
      &lt;item
          android:id="@+id/placeholder"
          android:icon="@drawable/ic_android_black_48dp"
          android:checked="false"
          android:title="Placeholder" /&gt;
      &lt;item
          android:id="@+id/placeholder2"
          android:icon="@drawable/ic_android_black_48dp"
          android:checked="false"
          android:title="Placeholder" /&gt;
  &lt;/group&gt;
  </code></pre>
  
  <p></p>
  
  <p>which adds a submenu which looks like this:
  <a href="https://i.stack.imgur.com/3xbh9.png" rel="noreferrer"><img src="https://i.stack.imgur.com/3xbh9.png" alt="enter image description here"></a></p>
  
  <p>However, I am unable to make this submenu look like this: (collapsable)</p>
  
  <p><a href="https://i.stack.imgur.com/wPfWl.png" rel="noreferrer"><img src="https://i.stack.imgur.com/wPfWl.png" alt="enter image description here"></a></p>
  
  <blockquote>
    <p>I now know that I need to implement an expandableListView, but I am not sure how this would fit in with the menu I have already created.</p>
  </blockquote>
  
  <p>Any advice is appreciated</p>