The active AccessibilityHierarchy for the screen reader. May be null if no hierarchy is active.
You need an active accessibility hierarchy to present any content to the user through the screen reader.
If the screen reader is off, there is no active hierarchy. If the screen reader is turned off on the device
while an active hierarchy is set, the active hierarchy is automatically set to null.
For all the supported platforms, refer to AssistiveSupport.
Throws PlatformNotSupportedException if the screen reader support is not implemented for the
platform and the code is not running in the Unity Editor.
When the active hierarchy is assigned, a notification is sent to the operating system that the screen changed
considerably. The notification is sent by calling IAccessibilityNotificationDispatcher.SendScreenChanged (with a null parameter).