Update connected clients when a downstream disappears.

Otherwise, if another downstream of the same type reappears, the
code would fire a callback with the previous list of clients.

Bug: 150644681
Test: atest TetheringIntegrationTests:EthernetTetheringTest --rerun-until-failure 100
Change-Id: I6b34ea747ae1831001077f44879bb6828dcecc96