Fix usb tethering switch back to none after off/on the screen

This CL adds conditions to check whether usb is connected or not.
If usb is not connected or tethering is not started don't update the
state.

Bug: 157709648
Test: make RunSettingsRoboTests -j56
Change-Id: Ifbf5900923419d6abf35834b196aab12601076e4