Add sampleSize to wallpaper_info.xml

Without that, the sample size is assumed to be 1 on reboot. Let's not
go in the details of what this breaks, let's just assume that this
breaks stuff.

Flag: ACONFIG com.android.windows.flag.multi_crop DEVELOPMENT
Bug: 328250071
Test: manual debug (unfortunately we don't have reboot tests)
Change-Id: If55c309c5c1a09a354c0c1149be3719229d53142