Summary:
React Native components need a mechanism to specify their value to assistive technologies. This PR adds the notion of accessibilityValueDescription-- a property which either contains a textual description of a component's value, or for range-based components, such as sliders and progress bars, it contains range information (minimum, current, and maximum).
On iOS, the range-based info if present is converted into a percentage and added to the accessibilityValue property of the UIView. If text is present as part of the accessibilityValueDescription, it is used instead of the range-based information.
On Android, any range-based information in accessibilityValueDescription is exposed in the AccessibilityNodeInfo's RangeInfo. Text which is part of accessibilityValueDescription is appended to the content description.
## Changelog
[GENERAL] [Change] - add accessibilityValuedescription property.
Pull Request resolved: https://github.com/facebook/react-native/pull/26169
Test Plan: Added two new accessibility examples to RNTester, one which uses text and another which uses range-based info in accessibilityValueDescription. Verified that they both behave correctly on both Android and iOS.
Differential Revision: D17444730
Pulled By: cpojer
fbshipit-source-id: 1fb3252a90f88f7cafe1cbf7db08c03f14cc2321
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