Register callback and receiver after tethering is created

Tethering service is created after boot complete which means most of
the services are ready before tethering. Once tethering register the
callback, callback event may come-in immediately. Make sure all of
tethering related object is created, then register the callback,
receiver, or listener.

Bug: 149965121
Test: atest TetheringTests
      manual on/off tethering

Change-Id: I3941a186770679e7b476073d774e2310e25e44c6