Group accessible views using the view hierarchy

Summary: In iOS when a parent UIView returns YES on [shouldGroupAccessibilityChildren](https://developer.apple.com/documentation/objectivec/nsobject/1615143-shouldgroupaccessibilitychildren), VoiceOver groups together the accessible children of the parent view, regardless of their position on screen. In iOS this defaults to NO.

Reviewed By: sammy-SC

Differential Revision: D23844265

fbshipit-source-id: eb99bf0873ccfd9fb196f8f7b6eafe055f6ae810

React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm

@@ -569,6 +569,11 @@ static NSString *RCTRecursiveAccessibilityLabel(UIView *view)
 
 #pragma mark - Accessibility Events
 
+- (BOOL)shouldGroupAccessibilityChildren
+{
+  return YES;
+}
+
 - (NSArray<UIAccessibilityCustomAction *> *)accessibilityCustomActions
 {
   auto const &accessibilityActions = _props->accessibilityActions;