Fabric: Using proper defaults in Yoga style props parsing

Summary:
@public
Previously, we amitted `default` argument in  `convertRawProp` functions assuming that all default values are equal to results of their default constructors (which was something between "wrong" and "lucky coincidence"). Now we use a `YGStyle` default value as a source of all semantic default values of all layout props/styles.

Reviewed By: mdvacca

Differential Revision: D9626469

fbshipit-source-id: 5cfc9c518772556f59da46f608181145cc744928