Make PrivateDnsConfiguration report event to binder directly

We have used function pointer to report the event to the binder
netd_listener for a while. Now make it talk to the binder service
directly.

Test: built, flashed, booted
      system/netd/tests/runtests.sh passed
      settings UI showed correct private DNS status
Change-Id: I20580efebb3113dfd87e8c368ac383ad85e009af