Remove PropTypes from Modal.js (#21279)
Remove PropTypes from Modal.js (#21279)

Summary:
This PR converts the Prop Types in `Modal` to Flow Types, and fills out the callback types a bit more.

Context Types are left in for now.
Pull Request resolved: https://github.com/facebook/react-native/pull/21279

Reviewed By: yungsters

Differential Revision: D10006795

Pulled By: TheSavior

fbshipit-source-id: ac885f2e5f068b0991009a9b1cbb3886e34941af

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