Refactor NetworkFactory.

Make NetworkFactory a concrete class and divide responsibilites between it and NetworkAgent.
Factory will track requests and by default give a single connect/disconnect api for ease
of use.  Then NetworkAgent is created and destroyed as needed with very simple logic.

Change-Id: I401c14a6e5466f2fc63b04219b97ff85bb9af291
(cherry picked from commit 9a17b9c5a256cb4bb14821c5ee89b03b99c045e8)