Non-functional code cleanup of ConnectivityService.

1. Remove ConnectivityService.findConnectionTypeForIface() as this can be done
   just as easily with supported APIs now.
2. Avoid making copies of Network objects as this precludes reuse of Network
   internals (e.g. socket factory, connection pool).

Change-Id: I52f92e35d769d8350471f485e408169608630082