re-factor parseTimeZone for coverity

:Release Notes:
Ensure that time-zone existence and opening works for same file.

:Detailed Notes:
- replaced stat() in file existence checks with actual fopen
- replaced stat() with fstat()
- hold opened file

:Testing Performed:
Verified on desktop with:
luna-send -n 1 -f luna://com.palm.systemservice/timezone/getTimeZoneRules '[{ "tz": "Europe/KievX" }]'
luna-send -n 1 -f luna://com.palm.systemservice/timezone/getTimeZoneRules '[{ "tz": "Europe/Kiev" }]'

:QA Notes:

:Issues Addressed:
[GF-52225] luna-sysservice: Coverity reports Medium and Low impact issues

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>

Change-Id: I5ed0e95e3cadceb7e9c67e8b11553bae2ea4e8bf
Reviewed-on: https://g2g.palm.com/3989
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: DCO Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>