ethernet: fix receiving NetworkOffer callbacks on stale object

unregisterNetworkOffer() does not execute synchronously, so it is
possible that NetworkOffer callbacks are received after the offer was
unregistered.

Test: atest EthernetManagerTest && atest EthernetNetworkFactoryTest
Bug: 171872016
Change-Id: I2c50b01176e4968c31f83148f1831b55f8b06908
2 files changed