Stop monitoring sockets until all sockets are unrequested

Now MdnsSocketProvider is stopped when there is no client request
left in NsdService, but this does not trigger
SocketCallback.onInterfaceDestroyed callbacks. If the network of
the socket is then lost while MdnsSocketProvider is not
monitoring, no callback will be fired. Users of the socket
(MdnsDiscoveryManager and MdnsAdvertiser) may keep using it
without ever getting notified. So ignore the stop and wait until
all sockets are unrequested. Then the socket destroy should be
notified to all users.

Bug: 267978487
Test: atest FrameworksNetTests
Change-Id: I7a8bb0550262fe397b91f1236a8dbca1cf2c7518