Hide tethering option when in a guest account

- Before this CL, users can share their mobile data through enabling
  tethering in the USB menu when in a guest account.
  It will cause main users to spend their money unexpectedly.

  This CL will hide the tethering option on the USB menu when in the
  guest account.
- Add test cases.

Bug: 166125765
Test: 1. make -j42 RunSettingsRoboTests. 2. Switch to a guest account
to verify whether the tethering option is hiding.

Change-Id: I7523b3f3c3a1372bb128a58a6a7fac973d27cfbe