Refactor support for setting custom accessibilityElements

Using the existing `accessibilityElements` ivar should be sufficient to
support custom accessibility elements, and thus this commit removes the
use of `customAccessibilityElements`.

Also, this commit adds a check to see if the node is loaded before one
can set custom accessibility elements on the node to avoid loading the view
unnecessarily.

Source/Details/_ASDisplayView.mm

@@ -78,7 +78,6 @@ @implementation _ASDisplayView
   BOOL _inPointInside;
 
   NSArray *_accessibilityElements;
-  NSArray *_customAccessibilityElements;
   CGRect _lastAccessibilityElementsFrame;
 
   _ASDisplayViewMethodOverrides _methodOverrides;

Source/Details/_ASDisplayViewAccessiblity.mm

@@ -245,7 +245,6 @@ static void CollectAccessibilityElementsForView(UIView *view, NSMutableArray *el
 
 @interface _ASDisplayView () {
   NSArray *_accessibilityElements;
-  NSArray *_customAccessibilityElements;
 }
 
 @end
@@ -257,8 +256,7 @@ @implementation _ASDisplayView (UIAccessibilityContainer)
 - (void)setAccessibilityElements:(NSArray *)accessibilityElements
 {
   ASDisplayNodeAssertMainThread();
-  _accessibilityElements = nil;
-  _customAccessibilityElements = accessibilityElements;
+  _accessibilityElements = accessibilityElements;
 }
 
 - (NSArray *)accessibilityElements
@@ -270,14 +268,15 @@ - (NSArray *)accessibilityElements
     return @[];
   }
 
-  if(_customAccessibilityElements != nil && _customAccessibilityElements.count > 0) {
-    return _customAccessibilityElements;
+  if (_accessibilityElements != nil) {
+    return _accessibilityElements;
   }
 
   if (_accessibilityElements == nil || ASActivateExperimentalFeature(ASExperimentalDisableAccessibilityCache)) {
-    _accessibilityElements = [viewNode accessibilityElements];
+    return [viewNode accessibilityElements];
   }
-  return _accessibilityElements;
+
+  return nil;
 }
 
 @end
@@ -286,6 +285,10 @@ @implementation ASDisplayNode (AccessibilityInternal)
 
 - (void)setAccessibilityElements:(NSArray *)accessibilityElements
 {
+  if (!self.isNodeLoaded) {
+    ASDisplayNodeFailAssert(@"Cannot set accessibilityElements since node is not loaded");
+    return;
+  }
   self.view.accessibilityElements = accessibilityElements;
 }