Bug 1026098 - Skip time and timezone setting when network time available. r=fcampo

please make this (together with 'USER' and 'SIM') a const for better readability. 
e.g ```
const TIMEZONE_NONE = 'NONE';
const TIMEZONE_NETWORK = 'NETWORK';
const TIMEZONE_SIM = 'SIM';
const TIMEZONE_USER = 'USER';
```