wallpaper: limit wallpaper width to sys.max_texture_size

Image wallpapers are created ~1.5x screen size.  On some devices this
may exceed the maximum supported texture size, which will either fail
allocation or fail to composite.

https://android-review.googlesource.com/95111

Bug 991597

Change-Id: I9948b09f6e00a724212e73d36901f2bbea42e2f4
Signed-off-by: houst0nn <houstonn@gmail.com>