Fix last NTP time-stamp check

:Release Notes:
Fixed false positive when checking whether that NTP date is still
valid.

:Detailed Notes:
- added more debug
- handle special case when m_lastNtpUpdate is set to zero which means
  there was no NTP date synchronization

:Testing Performed:
Tested in qemux86 build under VirtualBox

:QA Notes:

:Issues Addressed:
[GF-51427] luna-sysservice isn't invoking sntp

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Change-Id: If6e4034f0dc1c92296751f532cbd1657dc4ce8b2
Reviewed-on: https://g2g.palm.com/3955
Reviewed-by: DCO Verification
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Keith Derrick <keith.derrick@lge.com>