Make netd_event_listener_interface available to Tethering.

In AOSP master, tethering links to on netd-client, which links to
netd_event_listener_interface. Make netd_event_listener_interface
available to the tethering apex so we can sync the tethering code
from AOSP master to mainline-prod.

Partial cherry-pick of aosp/1535680.

Bug: 167645754
Test: m com.android.tethering
Merged-In: I4ab2006ec73fd00a636c73946269f84193198511
Change-Id: Icc2987bb39206f39546d19bc0264f8239ae68192