Let ConnectivityService control the socket closure

Netd currently calls maybeCloseSockets before adding/removing users for
network. The task should be moved from netd to CS. In this way, we can
handle WiFi lingering more easily in the future.

Test: cd system/netd; atest
Test: atest HostsideVpnTests
Test: atest FrameworksNetTests
Change-Id: Iaffd73b400e69417633551725bd69b574d1987ac