NAN: Support multiple instantiation per UID - introduce a client ID.

Add the concept of a client ID which is used to identify a specific
(one of several) clients in all API calls.

(cherry-pick of aa36f071,
 Change-Id: I43f13d51bee8b57c6d0ba1174168288b8d6c21c5)

Bug: 26962024
Change-Id: I0e0ddf37ea9800ae97115d6af384b54b49a40d74