ref(blank-page): destroy local track after mount

To kill componentWillMount, call destroyLocalTrack after mount.
Navigation to the blank page was synthetically forced and no
UI issues were noticed, possibly because destroyLocalTrack may
already be async so destruction may already have been occurring
after mount.

react/features/welcome/components/BlankPage.native.js

@@ -29,7 +29,7 @@ class BlankPage extends Component<Props> {
      * @inheritdoc
      * @returns {void}
      */
-    componentWillMount() {
+    componentDidMount() {
         this.props.dispatch(destroyLocalTracks());
     }