Allow disabling of Bluetooth tethering if DISALLOW_CONFIG_TETHERING in

on

When turning on the tethering service through Bluetooth connection
and then enabling the DISALLOW_CONFIG_TETHERING restriction from
TestDPC/User Restrinctions, it should run normally without any
incidents.

Without this change, this scenario would raise a SecurityException from
bluetoothTethering(false); (file: PanService)

Bug: 27936525
Test: manual
Change-Id: I2df490ac28f129b369f531d106f924a88d721433