framework: Allow specifying max wallpaper width

For some devices with very high-res screen (1920x1200), the wallpaper
width suggested by the launcher may be too large for the GPU and so
we need to limit it. (2048 for the above mentioned screen)

We don't need to limit the height since that's the height of the
screen.

Change-Id: I5b25bd931db0b2610e260921f28b92d8b0966aa6