Don't tear down existing cell connections.

NetworkFactories signals indicate if we're interested in making new
connections, but should not be used to tear things down.
ConnectivityService will communicate disconnects directly with NetworkAgents
when it wants them to tear down.

This fixes broken lingers and also potential score ties.

bug:15612739
Change-Id: Ib39ddadaac84898d080764afc365f09f7dce4ffa