[SPARK-32924] Make duration column in master UI sorted in the correct order

Author: Baohe Zhang

core/src/main/scala/org/apache/spark/deploy/master/ui/MasterPage.scala

@@ -309,7 +309,9 @@ private[ui] class MasterPage(parent: MasterWebUI) extends WebUIPage("") {
       <td>{UIUtils.formatDate(app.submitDate)}</td>
       <td>{app.desc.user}</td>
       <td>{app.state.toString}</td>
-      <td>{UIUtils.formatDuration(app.duration)}</td>
+      <td sorttable_customkey={app.duration.toString}>
+        {UIUtils.formatDuration(app.duration)}
+      </td>
     </tr>
   }