provide public hook to programatically turn on voiceover

Summary:
Changelog: [Internal]

https://fb.workplace.com/groups/rn.support/posts/6677051292343429

ax team is building a tool to extract information about the views for design reviewers, and RN has some AX information that is not working atm because of dependency on whether voiceover is on or not. so, this will give them the ability to programmatically set that field and hopefully be able to get accurate ax info

Reviewed By: ikenwoo

Differential Revision: D31010566

fbshipit-source-id: 4c8a33fce40266b270dd5994442c8472ca88f5dd