Enable scaling to fit various screen resolutions

Since the AGL HomeScreen of CES2018 assumes that the screen
resolution is 1080x1920px, the graphics of it partially
corrupted with others.

To fix this issue, now the AGL window manager automatically
scales size according to "scaling" value in setting.json

By default(even if "scaling" is not set), this scaling keeps
'fullscreen' aspect rate in area.db("aspect_fit")

User can select 3 options.
 - "aspect_fit" : Scale aspect rate of 'fullscreen' in area.db.(default)
 - "display_fit": Force to scale to display size.
 - "none" or others
                : Set size just as area.db

Bug-AGL: SPEC-1568
Bug-AGL: SPEC-1569

Change-Id: Ia08c0ebb2d71ae8f89a90088e181381c3ba3562d
Signed-off-by: Kazumasa Mitsunari <knimitz@witz-inc.co.jp>