Wait on IpClient#onQuit in StartedState#exit

This does make the entire operation synchronous, but also temporarily
blocks the handler thread (it is however what EthernetNetworkFactory
currently does). If that becomes an issue, we can consider adding a
StoppingState.

Test: mm
Change-Id: I799fb027d6eda4106633a047bf13674190ad0e68
1 file changed