Enable scaling to fit various screen resolutions

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 of HomeScreen to fit the resolution of the display.

By default, this scaling keeps HomeScreen's original aspect
rate (9:16). To ignore it, set the environment variable,
'HMI_SCALING_IGNORE_ASPECT'.
(E.g. set it in any file under /etc/afm/unit.env.d/)

Bug-AGL: SPEC-1138

Change-Id: Id11a07560fe254712aaab42018bfb4d1d87ad1df
Signed-off-by: Tadao Tanikawa <tanikawa.tadao@jp.panasonic.com>