The accessibility hierarchy that is currently accessible to screen readers.
An active AccessibilityHierarchy is required to make the content of an application accessible
to screen reader users.
To manage system resources efficiently, setting this property only takes effect if the screen reader is on,
where the value of AssistiveSupport.isScreenReaderEnabled is true. Similarly, when the
screen reader is turned off, so when the AssistiveSupport.screenReaderStatusChanged event is
sent with a false parameter or when AssistiveSupport.screenReaderStatusOverride is set to
AssistiveSupport.ScreenReaderStatusOverride.ForceDisabled, this property is automatically set
to null. You must set it each time the screen reader is turned on.
When this property is set, Unity notifies the screen reader of the new hierarchy by calling
IAccessibilityNotificationDispatcher.SendScreenChanged (with a null parameter).