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/BaseViewManager.java

@@ -179,6 +179,7 @@ public abstract class BaseViewManager<T extends View, C extends LayoutShadowNode
     final ReadableMap accessibilityState = (ReadableMap) view.getTag(R.id.accessibility_state);
     final String accessibilityHint = (String) view.getTag(R.id.accessibility_hint);
     final List<String> contentDescription = new ArrayList<>();
+    final ReadableMap accessibilityValue = (ReadableMap) view.getTag(R.id.accessibility_value);
     if (accessibilityLabel != null) {
       contentDescription.add(accessibilityLabel);
     }
@@ -205,6 +206,12 @@ public abstract class BaseViewManager<T extends View, C extends LayoutShadowNode
         }
       }
     }
+    if (accessibilityValue != null && accessibilityValue.hasKey("text")) {
+      final Dynamic text = accessibilityValue.getDynamic("text");
+      if (text != null && text.getType() == ReadableType.String) {
+        contentDescription.add(text.asString());
+      }
+    }
     if (accessibilityHint != null) {
       contentDescription.add(accessibilityHint);
     }
@@ -223,6 +230,18 @@ public abstract class BaseViewManager<T extends View, C extends LayoutShadowNode
     view.setTag(R.id.accessibility_actions, accessibilityActions);
   }
 
+  @ReactProp(name = ViewProps.ACCESSIBILITY_VALUE)
+  public void setAccessibilityValue(T view, ReadableMap accessibilityValue) {
+    if (accessibilityValue == null) {
+      return;
+    }
+
+    view.setTag(R.id.accessibility_value, accessibilityValue);
+    if (accessibilityValue.hasKey("text")) {
+      updateViewContentDescription(view);
+    }
+  }
+
   @Override
   @ReactProp(name = ViewProps.IMPORTANT_FOR_ACCESSIBILITY)
   public void setImportantForAccessibility(