siptransportbroker: avoid deadlock when stopping daemon

SipTransportBroker::shutdown triggers pjsip_transport_shutdown
that can call transportStateChanged synchronously. In this case,
transportMapMutex_ is locked, causing a deadlock on a single thread.
We don't care about the transport's map at this point as we are
destroying the transportbroker, so we can just ignore the state
changed here.

Change-Id: I180ff1807fe87d1c48d72851f17e94ae2f892823