Suppress deprecation warning in CallFragment.

The old signature of this method has been deprecated. However, the new
signature is only support API level 23+, so we can't really use it yet.

BUG=webrtc:6558

Review-Url: https://codereview.webrtc.org/2437593004
Cr-Commit-Position: refs/heads/master@{#14695}

webrtc/examples/androidapp/src/org/appspot/apprtc/CallFragment.java

@@ -129,6 +129,8 @@ public class CallFragment extends Fragment {
     }
   }
 
+  // TODO(sakal): Replace with onAttach(Context) once we only support API level 23+.
+  @SuppressWarnings("deprecation")
   @Override
   public void onAttach(Activity activity) {
     super.onAttach(activity);