Summary:
Exposing this enum is essentially useless and at worst is a runtime cost that isn't necessary by just using the string.
The value of this enum, as far as I understand it, is to enforce that only valid options are used. We can enforce this at build time with Flow.
I was able to migrate our codebase with a few Find and Replace for things like
```
resizeMode={Image.resizeMode.contain}
```
Reviewed By: yungsters
Differential Revision: D7983982
fbshipit-source-id: ddd7024023f8d2f01aad1fff6c8103983a1bec1a
870775ee73