Do not use TetherUtil.setWifiTethering

This functionality is replaced by ConnectivityManager.startTethering()
and ConnectivityManager.stopTethering().

Bug: 30147602
Test: Can toggle tethering on/off via all of settings, quick settings,
      and quick quick settings.
Change-Id: I4d190185c473237982b988f898e76d4997dc401c