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

Reapply "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.

(cherry picked from commit f8f3fbbd016d12f80c02e136ab4684da807792a1)

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.

Merged-In: Ib7ea8885cedc2a842ebd4487c8b366a6666996bc
Change-Id: I500e51fb44a93e943f8566b544e7ad4983ac6a6e