PanelFragmentContainerId
A panel fragment container ID.
When the system UI is recreated, the fragments are attach to a fragment container based on the original view ID. To simply recreations of the panel fragment containers with the correct view IDs, the view ID is generated and stored in the PanelSubContainerViewModel implementations.
Use PanelSubContainerViewModel.generatePanelFragmentContainerId to construct an instance of this class.