Resolve possible race condition in CellularNetworkService

There might be a race condition that before callback is stored
in mCallbackMap, complete message is back from mCi. Thus callback
is not retrived correctly when handling that message. This change
fixes that bug.

Bug: 73006796
Test: unit-test
Change-Id: I378928938f47b9ed9f9a6228db9931cc0a039171