ConnectivityService: move reportNetworkConnectivity to handler

This patch moves reportNetworkConnectivity onto the handler of
ConnectivityService.

This allows:
  - to inspect NetworkAgentInfo on the ConnectivityService handler,
    which is always more correct than doing so on a Binder thread.
  - to improve locking policies around NetworkAgentInfo.

Test: $ runtest frameworks-net
Bug: 37119619, 36902662
Change-Id: I49a765826e65c29a1995242290e5e7544112c94e