[NS A03] Remove mNetworkForRequestId

The goal of doing this is to remove the locks. Locking imposes
an ordering that is parallel to the program sequence ; often
it is what is needed, but the refactoring in progress needs to
change the traversal order of the requests without having
observable side-effects (or at least, very controlled ones).
The order is difficult enough to maintain when there is
only one. In this case, it is easy to remove the locks, and it
will simplify vastly the complexity of the refactoring by
removing the parallel ordering that would have to be maintained.

Bug: 113554781
Test: ConnectivityServiceTest
Change-Id: Ie4ae6a97053559e6cbac8230f2db3d35000b35a9