Fix an NPE in NsdManager

The NPE happens because NSD Manager doesn't always notify with a 'good'
notification for SERVICE_FOUND. It can get in a situation where a
SERVICE_FOUND is recevied from MDnsDs demon when processing StopDiscovery
on the messaging thread. When that happens, NsdService sends a message
to NsdManager with an invalid index of the listener.

The fix is twofold. First, we fix NsdService to not generate a message if
it doesn't have a good listener index. And second, we also fix NsdManager
to watch for invalid index.

Change-Id: I3d63af10bded13c72e8e437a1ebf74a666760432