Release the interface request when Ethernet tethering is stopped.

When the Ethernet interface becomes unavailable (e.g., because
the cable was unplugged or the interface was removed), or when
setEthernetTethering(false) is called, release the Ethernet
interface request.

This ensures that:
- The Ethernet interface immediately becomes available for use in
  client mode.
- If an interface later becomes available, tethering is not
  automatically started. This is consistent with what happens for
  other downstream types such as wifi and USB. Evey time one of
  those downstreams goes down, tethering is stopped and will not
  be restarted.

Test: manual
Bug: 148824036
Change-Id: Iaf85e800569f2e08c39f7ebb96f8aa34f6e53133