[NAN] Add master switch to enable/disable usage of NAN APIs.

Add master switch to enable/disable usage of NAN. Tie into Wi-Fi
state machine (to enable/disable based on Wi-Fi enable/disable).
Dispatch broadcasts when NAN status changes.

Bug: 27698011
Change-Id: Iea36388443ff8d0dc26a280220022c6bdba6a0c8
3 files changed