-[RCTAccessibilityManager announceForAccessibility:] calls into AppKit from background thread

[nakambo]

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

Specifically, NSAccessibilityPostNotificationWithUserInfo. Other methods such as getCurrentHighContrastState provide cached state (cached on main thread via observeValueForKeyPath et al). Some other methods such as setAccessibilityFocus dispatch to the main thread.

Not doing so could lead to AppKit further firing selectors on UI logic in app code and lead to downstream issues.

Test Plan

I've verified that original badness I observed in Office that led to discovering this no longer reproduces. Announcement posts in Accessibility Inspector.

[github-actions]

Fails

🚫

📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. CATEGORY may be: General macOS iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.

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