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

if we get rid of 'userSelected' and change https://github.com/Moz-TedC/gaia/blob/bug-1026098-fix-attempt-2/shared/js/tz_select.js#L173 and https://github.com/Moz-TedC/gaia/blob/bug-1026098-fix-attempt-2/shared/js/tz_select.js#L177 to `source = 'USER';`, we would depend only on one variable here, so we could make
```
onchangeTZ(getTZInfo(), {
  'changedByUser': source === 'USER',
  'needsConfirmation': source !== 'NETWORK'});
}
```
...or even just return the `source` and let `initSelector` check it (but I think is better to keep it divided in two parameters for better readability)
Does this make sense to you? Wdyt?