Refactor Drupal\Core\Template\AttributeArray to use ArrayObject and implement AttributeValueInterface

Problem

As noted in the parent issue, extending from ArrayObject is usually faster and cleaner than implementing the ArrayAccess interface in userland code. For AttributeArray to do this it must implement the AttributeValueInterface since multiple inheritance isn't possible.

After this patch the AttributeArray class is cut in half by code lines count with no loss of functionality. As with its parent it picks up the \Serializable interface throught this extension.