ref(audio-picker): fetch audio devices after mount

Per react migration docs, initially fetching external data is
recommended to be done in componentDidMount.

react/features/mobile/audio-mode/components/AudioRoutePickerDialog.js

     state = {
         /**
          * Available audio devices, it will be set in
-         * {@link #componentWillMount()}.
+         * {@link #componentDidMount()}.
          */
         devices: []
     };
      *
      * @inheritdoc
      */
-    componentWillMount() {
+    componentDidMount() {
         AudioMode.getAudioDevices().then(({ devices, selected }) => {
             const audioDevices = [];