iOS: Fixed some props conversion errors

Summary:
* numbers in JS are doubles in native land, since there's no notion of int or int64 in JS - so simply convert numbers to int instead of assuming it's int
* the parsing of Yoga props with `'...%'` string value has a bug: it should be copying the number instead of the `%`

Reviewed By: shergin

Differential Revision: D8370873

fbshipit-source-id: 44e9e3f0530c000c963e8e9ca66e8b0a48d80bcd