Add accessibilityElementsHidden prop

Summary:
Allow iOS to have similar accessibility functionality to Android. This PR exposes the `accessibilityElementsHidden` property on iOS which is similar to Android's `importantForAccessibility="no-hide-descendants"`

I didn't see any existing examples for testing native props being passed through, but I did add an example to the RNTester app. I've attached some screenshots using the Accessibility Inspector to verify the property was correctly passed through.

![a](https://user-images.githubusercontent.com/603528/34998153-50e66776-faac-11e7-826d-1445a6813929.png)
![b](https://user-images.githubusercontent.com/603528/34998158-535a7420-faac-11e7-80d4-992fb7cd82dd.png)

I've updated the website with appropriate documentation.

facebook/react-native-website#141

![screen shot 2018-01-16 at 10 23 50](https://user-images.githubusercontent.com/603528/34998202-6f2f39a6-faac-11e7-8651-0cfe8e037a30.png)
![screen shot 2018-01-16 at 10 23 59](https://user-images.githubusercontent.com/603528/34998205-711d6f94-faac-11e7-974d-54340c72fce4.png)

[IOS] [FEATURE] [View] - Added accessibilityElementsHidden property
Closes #17627

Differential Revision: D6806444

Pulled By: hramos

fbshipit-source-id: 50d31fdb92f4c59ae9355b019c422418b2e6cc24

Author: aputinski

Libraries/Components/View/ViewPropTypes.js

@@ -55,6 +55,7 @@ export type ViewProps = {
   importantForAccessibility?: 'auto'| 'yes'| 'no'| 'no-hide-descendants',
   accessibilityTraits?: AccessibilityTrait | Array<AccessibilityTrait>,
   accessibilityViewIsModal?: bool,
+  accessibilityElementsHidden?: bool,
   onAccessibilityAction?: Function,
   onAccessibilityTap?: Function,
   onMagicTap?: Function,
@@ -173,6 +174,16 @@ module.exports = {
    * See http://facebook.github.io/react-native/docs/view.html#accessibilityviewismodal
    */
   accessibilityViewIsModal: PropTypes.bool,
+  
+  /**
+   * A value indicating whether the accessibility elements contained within
+   * this accessibility element are hidden.
+   *
+   * @platform ios
+   * 
+   * See http://facebook.github.io/react-native/docs/view.html#accessibilityElementsHidden
+   */
+  accessibilityElementsHidden: PropTypes.bool,
 
   /**
    * When `accessible` is true, the system will try to invoke this function

RNTester/js/AccessibilityIOSExample.js

@@ -55,6 +55,9 @@ class AccessibilityIOSExample extends React.Component<{}> {
           accessible={true}>
           This text component's accessibilityLabel is set explicitly.
         </Text>
+        <View accessibilityElementsHidden={true}>
+          <Text>This view's children are hidden from the accessibility tree</Text>
+        </View>
       </View>
     );
   }

React/Views/RCTViewManager.m

@@ -113,6 +113,7 @@ - (RCTShadowView *)shadowView
 RCT_REMAP_VIEW_PROPERTY(accessibilityLabel, reactAccessibilityElement.accessibilityLabel, NSString)
 RCT_REMAP_VIEW_PROPERTY(accessibilityTraits, reactAccessibilityElement.accessibilityTraits, UIAccessibilityTraits)
 RCT_REMAP_VIEW_PROPERTY(accessibilityViewIsModal, reactAccessibilityElement.accessibilityViewIsModal, BOOL)
+RCT_REMAP_VIEW_PROPERTY(accessibilityElementsHidden, reactAccessibilityElement.accessibilityElementsHidden, BOOL)
 RCT_REMAP_VIEW_PROPERTY(onAccessibilityAction, reactAccessibilityElement.onAccessibilityAction, RCTDirectEventBlock)
 RCT_REMAP_VIEW_PROPERTY(onAccessibilityTap, reactAccessibilityElement.onAccessibilityTap, RCTDirectEventBlock)
 RCT_REMAP_VIEW_PROPERTY(onMagicTap, reactAccessibilityElement.onMagicTap, RCTDirectEventBlock)