Clear calling identity in registerNetworkAgent.

Much of registerNetworkAgent calls internal ConnectivityService
methods which generally assume that they are not processing an
IPC and are running under the system's calling identity.

However, only the call to makeNetworkMonitor is run with caller
identity cleared. Expand the scope of clearing the caller
identity over the creation of the nai.

Bug: 173331190
Test: passes existing tests in ConnectivityServiceTest
Change-Id: Icad28601a612fb5e1ed0451ec9e2066f4e766d0e