Make ConnectivityServiceTest a bit more readable.

1. Make TestNetworkCallback a bit smarter and rename it to
   SingleUseNetworkCallback. This allows us to get rid of all the
   calls to TestNetworkCallback#getConditionVariable.
2. Delete the commented out code that used to test a
   ConnectivityService model that has not been used since KK.
3. Remove unused imports, etc.

Bug: 22606153
Change-Id: I81a2d0b970d19e5f4515490d8c2f88d416445fa1