Change scan interval to 15 seconds

:Release Notes:
Changing the default scan interval from 30 to 15 seconds.

:Detailed Notes:
After some more discussion and the aim to have new APs detected very fast
we decided to lower the scan interval to 15 seconds.

:Testing Performed:
Verified with a local AP that it shows up in the list of networks after
15 seconds when it gets enabled.

:QA Notes:

:Issues Addressed:
[GF-35360] Change default scan interval to 15 seconds

Open-webOS-DCO-1.0-Signed-off-by: Simon Busch <simon.busch@lge.com>

Change-Id: Ib27d5952571ee4aab2cf33927c507385b0157733
Reviewed-on: https://g2g.palm.com/3677
Reviewed-by: Build Verification
Reviewed-by: Sapna Todwal <sapna.todwal@lge.com>
Tested-by: Sapna Todwal <sapna.todwal@lge.com>