iOS: Use official method to get root view controller
iOS: Use official method to get root view controller

The current way of obtaining the root view controller  appears to not be working in RN 0.59.10. This new approach uses an official [method](https://github.com/facebook/react-native/blob/91681016e84cdb99bd823c937762a0d9974e56fe/React/Base/RCTUtils.h#L87) provided by React Native.

Elyx0/react-native-document-picker,Elyx0/react-native-document-picker,Elyx0/react-native-document-picker,Elyx0/react-native-document-picker,Elyx0/react-native-document-picker