Called when the panel has been dismissed from the system UI, either through user interaction or other events. This triggers the end of the life of a panel; it will be removed from the frontend, causing onRemovedFromFrontend to be called and preventing it from being shown again in the system UI.

This should only be called from components that already handled the dismissal process, such as a system UI that animated a panel away after a user swiped it away. For triggering the dismissal initially, dismiss should be used instead.