Implemented AccessibilityInfo.announceForAccessibility

[graytmatterMS]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

When the MacOS fork was made all the AccessibilityInfo functionality was stubbed out because the iOS Accessibility APIs are different from the MacOS APIs. I've filled back out the functionality for announceForAccessibility.

Changelog

[MacOS] [Added] - Implemented AccessibilityInfo.announceForAccessibility

Test Plan

1. Run RNTester
2. Go to the accessibility page
3. Scroll to Announce for Accessibility (the second to last entry)
4. Toggle VoiceOver in settings.
5. Move VoiceOver Focus to "Announce for Accessibility"
6. Hit [Space]

• You should see/hear Voice Over announce "Announcement Test"

Microsoft Reviewers: Open in CodeFlow

[graytmatterMS]

I've only been able to repro the test failure once, despite running the tests 5-ish times.