Summary:
Expose native pointer thru an interface function for YogaConfig (its package private to `com.facebook.yoga` namespace),
This way we can make later on YogaConfig a pure abstract class.
Plus, it makes sure external users don't modify the pointer
Reviewed By: SidharthGuglani
Differential Revision: D17266401
fbshipit-source-id: f39b488cea0b73bc3578bb3aa90ab00139bf9271