Summary:
Remove usage of the legacy context API in Modal in favor of the new context API. Closes https://github.com/facebook/react-native/issues/28103
Pull Request resolved: https://github.com/facebook/react-native/pull/29002
Test Plan: Run RNTester app and test the Modal example. Also add a `console.warn` to make sure we get the correct context value.
Differential Revision: D21793993
Pulled By: TheSavior
fbshipit-source-id: de2a30cbd46507bfa73a563d2429c5a7f0e320c9