[NAN] Register WIFI_NAN_STATE_CHANGED as protected broadcast [DO NOT MERGE]

Broadcast can be registered protected (as in this CL) in which case they
are restricted to system apps. Broadcasts sent by the System Service must
be protected (so no one else can fake them).

The WIFI_NAN_STATE_CHANGED is sent by the System Service and no other apps
should be able to send them (e.g. to deny access to NAN to other apps).

Bug: 31811154
Test: build & run - enable/disable Wi-Fi to trigger bug
Change-Id: I9591af930b60842130809cdb37388be515fe6dc3