applets/devicenotifier: handle loop devices
Reason for the change
Properly handle loop devices. They are removable but do not have a removable parent, which breaks the Mount and MountAndOpen actions. This is also why the “Safely removable” message should not be shown.
Also, if a loop device is already mounted, do not allow it to be unmounted, as it may be used by the system.
Test plan
Screenshots or screen recordings
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