Return the registry in registerNetworkAgent.

This is a resubmit of
I0fd90716b9f5501bdcdc7ee5165321737a0076ae
which did not account for testing of this private method
in Telephony tests.

Also make the network agent registry two way to eliminate
reordering possibilities.

Test: ConnectivityCoverageTest, CtsNetTests
Change-Id: I9ca025b1369dcb1202f906d5cff79e3d935b40da
9 files changed