The hierarchyManager will manage the competition between modules with "top level" UI elements.

hierarchyManager.registerHierarchy(module);
hierarchyManager.unregisterHierarchy(module);

The module responsible for displaying "top level" UI elements to the user should:
* Implement the isActive function
* Dispatch activated/deactivated events when active state changes
* (Optional) Implement getActiveWindow if the module is a window manager
* (Optional) Implement setHierarchy which will be invoked by hierarchy manager when the top most module is changed.

Any module who wants to get focus should do:

System.request('focus', module);
If you are the top most module, HierarchyManager will then invoke module.setHierarchy(true). Otherwise the request will just be ignored.

The hierarchyManager will replace the visibilityManager, including the a11y and audio-channel support.

== List of competing modules ==
* AttentionWindowManager
* SecureWindowManager
* LockScreenWindowManager
* AppWindowManager
* UtilityTray
* SIM PIN Dialog
* Dev tools Dialog
* Rocketbar / Search
* Cards View
* Bluetooth Pairing Dialog ?