Summary:
Adds a flag useVanillaJNI in YogaConfig to determine whether to use FbJNI or JNI.
Currently default is set to false.
We will experiment based on this flag once all code have been migrated.
Reviewed By: Andrey-Mishanin
Differential Revision: D17601703
fbshipit-source-id: 377a5bd2a6f8a7584e84932e87fa7044d8165efd