Adjust NICE value of db8

:Release Notes:
Adjust NICE value of db8 from 0 to -5

:Detailed Notes:
In order to improve UX, we need to adjust nice values of system services.
Below services are important so we need to give more priority.
- Rendering services
- Media services
- Core services

Here is a suggestion.
- LS2 : -5
- DBUS : -5
- GM : -15
- umediaserver : -10
- LSM : -15
- audiod : -5
- db8 : -5
- SAM : -5
- showcase : -5
- MaliitServer : -5
- WebAppMgr : -5
- QtWebProcess : -5

:Testing Performed:

:QA Notes:

:Issues Addressed:
[GF-36804] Adjust NICE values for all processes

Open-webOS-DCO-1.0-Signed-off-by: Sangwoo Kang <sangwoo82.kang@lge.com>

Change-Id: I30b8673bdb7893424bd1836bd528e3a602a926ca
Reviewed-on: https://g2g.palm.com/3395
Reviewed-by: Build Verification
Reviewed-by: Lokesh Kumar Goel <lokeshkumar.goel@lge.com>
Reviewed-by: Sangwoo Kang <sangwoo82.kang@lge.com>
Tested-by: Sangwoo Kang <sangwoo82.kang@lge.com>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>