Update x-android-system-characteristics.xsd

I am Srikanth Pai from Samsung Research India, Bangalore. Our team has added a telephony test to the existing latest schema. 
This will provide access to the telephony parameters on the device. SCAP can use this to test the telephony states. 

Telephony test can check for the following three parameters,

- network_type : SCAP can use this test to find out whether the device supports a network type and also to find all available network types supported by the device.
- sim_country_iso : This is a two letter country code mainly used to find the details of the country where the device is operated from.
- sim_operator_code : Operator Code consists of a device network code and a device country code. SCAP can use this code to find details of the device's network operator and whether it is in the list of allowed operators in that particular country or not.