TetherUtil: Use ConnectivityManager for tethering

In the past, enabling the SoftAP would cause tethering to happen because
of some unfortunate side effects.  This is no longer the case, and
using WifiManager for this purpose is not a good idea.

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

Change-Id: Iebbfc6c48b3e8e3fa52d235594ded3502f5801a3