[TNU01] Add Tethering notification updater

There are lots of Carrier/OEM requests for tethering
notification customization. So add a new tethering notification
updater class which can let OEM customize the behavior they
wanted.

Bug: 122085773
Test: atest TetheringTests
Change-Id: I7faacde7ac84e93ea0dfe03dd33d2cc41c589225