Notify the user and turn off tethering when the service is disallowed.

Added UserRestrinctionListener for turning the service off one the
DISALLOW_CONFIG_TETHERING is on into Tethering.  Added notification
about
tethering being turned off. Also added Unit Tests to test the
functionality
of the UserRestrictionListener added.

Bug: 27936525
Test: Turn the tehering service on (either wifi, usb or bluetooth).
Automatically the system should send a notification about the service
being active.Close settings from recents. From TestDPC User Restrictions
switch on DISALLOW_CONFIG_TETHERING. The tethering should be turned off
and a notification should appear informing that the service is inactive.

Change-Id: Ib7ea8885cedc2a842ebd4487c8b366a6666996bc