Fix BoxShadow support for platform colors (#46753)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46753

Colors can be more than just ints, so we need to use `ColorPropConverter` to convert it correctly. For ViewManagers, this happens automatically through the ReactProp annotation, but since this is a nested field, we need to manually apply it.

Changelog: [Android][Fixed] BoxShadow now supports platformColor.

Reviewed By: fabriziocucci

Differential Revision: D63693048

fbshipit-source-id: 26b1da24db1b6f319af8cef6f62e4739c3f65ee3