sidebar: Hide format for unmounted MTP/GPhoto2 volumes

Currently, the Format action is offered also for the MTP volumes when they are unmounted. This is a regression from the commit ab1d3019. This is because the uri variable is NULL for unmounted volumes. This amoung other causes critical warnings when opening popover menu for unmounted volumes. Let's check that the activation root is native file instead to avoid the mention issue. This will also correctly hide the Format action also for GPhoto2 devices that are mentioned in the original commit, but unhandled currently.