Add accessibilityValueDescription support. (#26169)

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

ReactAndroid/src/main/java/com/facebook/react/uimanager/ViewProps.java

@@ -141,6 +141,7 @@ public class ViewProps {
   public static final String ACCESSIBILITY_ROLE = "accessibilityRole";
   public static final String ACCESSIBILITY_STATE = "accessibilityState";
   public static final String ACCESSIBILITY_ACTIONS = "accessibilityActions";
+  public static final String ACCESSIBILITY_VALUE = "accessibilityValue";
   public static final String IMPORTANT_FOR_ACCESSIBILITY = "importantForAccessibility";
 
   // DEPRECATED