core/device: do not downgrade device state if it is already enumerated

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]