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: atest HostsideVpnTests
Test: atest FrameworksNetTests
Change-Id: Icf8125e8552c89da367a67f48611ed193a1a343d