Summary:
This diff is a follow up of D28360679 (https://github.com/facebook/react-native/commit/e3367354cc93f17b830ed8dc601dff5e87748dd1), here we refactor the access of ReactFeatureFlags from C++ to use methods instead of fields
changelog: [internal] internal
Reviewed By: JoshuaGross
Differential Revision: D28362066
fbshipit-source-id: caed5e7fddeb6c0d9846fb037152befa8f1ed5c2