[NAN] Separate ATTACH from IDENTITY_CHANGE callbacks.

Provide separate mechanism to attach with a request for identity
change listener.

(cherry-pick of commit 78e33f277eff5183f4525ce2a699acf2ad848177)

Bug: 31470256
Test: unit tests & integration (sl4a) tests.
Change-Id: I26b3bf4e904f3c3822578185fa55e2de5e63b71b
6 files changed