Add a carrier config to block tethering.

When KEY_CARRIER_SUPPORTS_TETHERING_BOOL=false, it will behave as if
entitlement check failed and a system notifcation shows up if user
tries to turn on tethering from Settings UI.

Bug: 161206517
Test: manual
Change-Id: I78da390ecb0c8f4f68285ef16da80653f136c11e
3 files changed