[NAN] Add Handler to publish/subscribe per API review

All callback registrations must have handlers.

(cherry-pick of commit 9b04fd2475f5ebecb81e75a8272cf410ba3f5854)

Bug: 31470256
Test: unit tests & integration (sl4a) tests.
Change-Id: I5ecc04f997e45b76710f526f378e4cf6e0e28fb9
2 files changed