Check the timezone availability before setting time zone

:Release Notes:
Check if timezone data is present before creating symlink to it. If
it's missing, fall back to the UTC.

:Detailed Notes:

:Testing Performed:
Build succeeded.
Tested on device with the following command:
 $ luna-send -n 1 luna://com.palm.systemservice/setPreferences '{"timeZone":{"ZoneID": "Europe/Kiev"}}'

:QA Notes:

:Issues Addressed:
[GF-44692] luna-sysservice shouldn't accept wrong time zones

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: I1fcbde653faa800fc8c198e81c20ddfc9c13231e
Reviewed-on: https://g2g.palm.com/3784
Reviewed-by: Build Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>