core/device: do not downgrade device state if it is already enumerated
    
    On switching root, a device may have a persistent databse. In that case,
    Device.enumerated_found may have DEVICE_FOUND_UDEV flag, and it is not
    necessary to downgrade the Device.deserialized_found and
    Device.deserialized_state. Otherwise, the state of the device unit may
    be changed plugged -> dead -> plugged, if the device has not been mounted.
    
    Fixes #23429.
    
    [mwilck: cherry-picked from #23437]
    
    (cherry picked from commit 4fc69e8a0949c2537019466f839d9b7aee5628c9)