Bug 1621848 - Fix setMessageDelegate API change in example. r=aklotz

Differential Revision: https://phabricator.services.mozilla.com/D66665

--HG--
extra : moz-landing-system : lando

mobile/android/examples/messaging_example/app/src/main/java/org/mozilla/geckoview/example/messaging/MainActivity.java

@@ -61,7 +61,7 @@ public class MainActivity extends AppCompatActivity {
             return null;
         });
 
-        session.setMessageDelegate(extension, messageDelegate, "browser");
+        extension.setMessageDelegate(messageDelegate, "browser");
 
         session.open(sRuntime);
         view.setSession(session);