Trebuchet: Force reload wallpaper dimension on screen size change

This ensures that the wallpaper dimensions are adjusted when a screen size change occurs,
for example when the expanded desktop feature is activated on devices without hard keys.

Change-Id: I4be96034a609afb2dd18e346b04ffacfed4e7043
JIRA: CYAN-790
Issue: https://jira.cyanogenmod.org/browse/CYAN-790
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>