Prevent crash in NetworkManagementServiceTest#shutdown()

The shutdown method in NetworkManagementServiceTest was trying to close
the local socket on the test side, causing the NativeDaemonConnector
internal to NetworkManagementService to sometime crash due to the
output stream on NetworkManagementService side to throw on pending
reads.

The correct fix would be to shutdown the NativeDaemonConnector
inside NetworkManagementService and implement NetworkManagementService's
shutdown method, however there is no way to cleanly close a
NativeDaemonConnector.

Instead, this patch doesn't do any cleanup of the listening socket, the
test local socket, and its output stream. These objects' resources get
eventually collected by the system when the test process exits.

Test: runtest frameworks-net
Change-Id: I72c9aa43403754b55e9d23bf4f3ba8b7b4a3e10a