correctly validate supplied date

:Release Notes:
fix leap year detection and streamline date validation

:Detailed Notes:
1. Current logic for leap year detection uses modulo 4, which is not
correct.
2. In order to fix this and to streamline validation of supplied date,
this commit uses glib's g_date_valid_dmy() function.

:Testing Performed:
Compile tested on local machine

:QA Notes:

:Issues Addressed:
[GF-7133] sleepd: Doesn't know how to recognize a leap year

Open-webOS-DCO-1.0-Signed-off-by: Ameya Palande <ameya.palande@lge.com>

Change-Id: I805641aa233116b774149015a60fbe8a954fcd14
Reviewed-on: https://g2g.palm.com/2988
Reviewed-by: Build Verification
Reviewed-by: Ameya Palande <ameya.palande@lge.com>
Tested-by: Ameya Palande <ameya.palande@lge.com>
Reviewed-by: Keith Derrick <keith.derrick@lge.com>