[RN] Use a more resilient way for testing if we are on React Native

navigator.product is read-only, so we don't run into the risk of it being
changed.

react/features/app/middleware.js

     // abstraction. Because of the first point, I'm leaving the second point
     // unresolved to attract attention to the fact that the following needs more
     // thinking.
-    if (navigator.userAgent.match(/react[ \s-]*native/i)) {
+    if (navigator.product === 'ReactNative') {
         // Create/destroy the local tracks as needed: create them the first time
         // we are going to render an actual route (be that the WelcomePage or
         // the Conference).

react/features/base/config/reducer.js

  */
 function _getInitialState() {
     return (
-        navigator.userAgent.match(/react[ \s-]*native/i)
+        navigator.product === 'ReactNative'
             ? INITIAL_RN_STATE
             : INITIAL_NON_RN_STATE);
 }