Create a cross-platform accessibility example module

[marcmulcahy]

Summary

This PR divides the accessibility tests into two activities-- a cross-platform activity for accessibility features which are expected to work on all platforms, and platform specific tests for Android and iOS.

We believe that most, if not all, accessibility features should be cross-platform, with fallback implementations where the underlying concept doesn't exist on a particular platform.

This division of the tests makes it clearer for developers which features are expected to work on all supported platforms, and which are platform-specific.

Changelog

[CATEGORY] general, Android, iOS
[TYPE] change

Message

Refactor the RNTester accessibility activities to better represent where the features are expected to work. Moves all cross-platform tests to AccessibilityExample.js, Android-specific tests to AccessibilityAndroidExample.android.js, and iOS-specific tests to AccessibilityIOsExample.ios.js.

Test Plan

Verified that tests work on iOS with VoiceOver and Android with TalkBack.

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[analysis-bot]

Code analysis results:

• eslint found some issues. Run yarn lint --fix to automatically fix problems.

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📖	📋 Changelog Format - Did you include a Changelog? A changelog entry has the following format: [CATEGORY] [TYPE] - Message.

Generated by 🚫 dangerJS against 1481c5a

[marcmulcahy]

changelog

[general] [change] - Re-factor accessibility tests into cross-platform and platform-specific tests.

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[cpojer]

This is a great start to improve our accessibility story. Thank you so much!

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @xuelgong in 1889b79.

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