Avoid leaking sockets.

When a DNS lookup fails, we end up creating a datagram socket
but never properly close it.  This wastes limited valuable file
descriptors and causes CTS test failures.  Generally, we should
ensure that sockets or files are opened in a try block, and closed
in a finally block.

Bug: 3276283
Change-Id: I3432f3bc061268c7367948230f6edbdfcec55892