rn: polyfill callstats

Instead of bundling it in lib-jitsi-meet, which unnecessarily increases
lib-jitsi-meet's bundle size, polyfill it here so it's available in the global
scope, just like the web does.

react/features/base/lib-jitsi-meet/native/polyfills-browser.js

 
     // WebRTC
     require('./polyfills-webrtc');
+
+    // CallStats
+    //
+    // Required by:
+    // - lib-jitsi-meet
     require('react-native-callstats/csio-polyfill');
+    global.callstats = require('react-native-callstats/callstats');
 
     // XMLHttpRequest
     if (global.XMLHttpRequest) {