Properly gate platform specific code in Alert
Properly gate platform specific code in Alert

Summary:
In the past, Alert and AlertIOS were separate modules and both were shipped on Android even though only Alert can be used there. We unified the two modules but it meant that the code for both was still shipped on both platforms.

This diff changes it so that platform specific code is gated in a `Platform.OS` block, which means that the unnecessary code per platform is being stripped out. I'm not looking to win a beauty contest with the code in this module - we have barely touched it in years and I don't think we will touch it anytime soon, so I just merged the sub-classes (which only had static methods anyway) directly into the main class. I'm aware I could make two separate files with platform extensions but that ends up being more code here and it seems straightforward enough to me.

Reviewed By: TheSavior

Differential Revision: D14057404

fbshipit-source-id: 1ae1d227a39d76de9779b3db62960cca46e9b75c

myntra/react-native,hoangpham95/react-native,exponent/react-native,arthuralee/react-native,janicduplessis/react-native,exponent/react-native,hoangpham95/react-native,janicduplessis/react-native,hoangpham95/react-native,exponent/react-native,pandiaraj44/react-native,exponentjs/react-native,hammerandchisel/react-native,janicduplessis/react-native,hammerandchisel/react-native,pandiaraj44/react-native,myntra/react-native,exponent/react-native,arthuralee/react-native,javache/react-native,hammerandchisel/react-native,hammerandchisel/react-native,hoangpham95/react-native,javache/react-native,facebook/react-native,exponentjs/react-native,arthuralee/react-native,myntra/react-native,myntra/react-native,hammerandchisel/react-native,facebook/react-native,hoangpham95/react-native,hoangpham95/react-native,exponent/react-native,hammerandchisel/react-native,janicduplessis/react-native,arthuralee/react-native,exponentjs/react-native,hoangpham95/react-native,facebook/react-native,javache/react-native,exponent/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,myntra/react-native,exponentjs/react-native,javache/react-native,javache/react-native,arthuralee/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,pandiaraj44/react-native,exponent/react-native,hammerandchisel/react-native,pandiaraj44/react-native,exponentjs/react-native,facebook/react-native,pandiaraj44/react-native,pandiaraj44/react-native,facebook/react-native,myntra/react-native,myntra/react-native,hammerandchisel/react-native,hoangpham95/react-native,exponentjs/react-native,exponentjs/react-native,exponentjs/react-native,facebook/react-native,facebook/react-native,myntra/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,myntra/react-native,exponent/react-native,pandiaraj44/react-native,javache/react-native,janicduplessis/react-native