…ed the re-definition of the property.

With this refactor, the field can now be used as a single access point into the accessibility elements of a view. Also, removing the re-definition of the property in _ASDisplayViewAccessibility.h enables us to make use of the field and its associated helper methods directly from the `UIAccessibilityContainer` API rather than rolling our own implementation.

Also added assertions that the getter and setter for the accessibilityElements
property are used only on the main thread.