Base panel type for media-related Frontends.
Media task panels may be tied to a certain source, for example to browse its content, or not, such as a generic media settings panel. When a media task panel is made to operate on or with a media source, it will have a non-
null sourceId, and
mutableMediaItem can be used to store additional state by adding Bundle fields to it, since the framework will re-create panels with the same mutableMediaItem; for example, to keep track of the activation state of a feature when closing and reopening the stock media frontend.
Note: The mediaItem is used by the stock media frontend to display navigation breadcrumbs.