Disallow guest user from changing Wifi settings

Disallow existing and newly created guest users from
changing Wifi settings.

BUG: 27411179
TEST: Flashed device, switched to existing guest user, and verified
      that Wifi settings are disabled.
TEST: Flashed device, created new guest user, and verified that Wifi
      settings are disabled.

Change-Id: Ia1bf4cce0369017b62f69d317c7ab2e30e3949b3