Fix "USB tethering" will auto turn on then off when turn off "USB tethering"

This CL only updates the "USB tethering preference" when
onTetheredInterfacesChanged() is invoked to avoid the
"USB tethering preference" being changed when received
non-tethering change intent.

Bug: 185590309
Test: make -j42 RunSettingsRoboTests
Change-Id: Ifed9f5c7edef670eeb21baf3841999382ee10ff2
2 files changed