Close netlink socket when shutting down IpReachabilityMonitor

This forces the NetlinkSocketObserver thread to exit quickly, rather
than lingering until the next random netlink neighbor multicast message
arrives.

Additionally, add a small unittest to verify that multiple calls to
NetlinkSocket#close() are safe.

Change-Id: I101730fad7eee72f9c6e8a7e7bd10c634f2ceab4