This is a preliminary step to removing mNetworkForRequestId, whose
role could be better managed by storing the network inside the NRI.
This serves two purposes :
1. It is a sanity check. Those functions should never be called out
of the handler thread, and if they are it's a bug.
2. It will serve to prove the followup changes are correct.
Bug: 113554781
Test: ConnectivityServiceTest
Change-Id: If29066839ad640121d33f231abdd4f37d0ad3fd5
4deee63037