Fix window cropping issue.

Comparing mLayer to mSystemDecorLayer no longer makes sense since mLayer
is always zero. The code was always falling into this check and causing
the crop to be calculated incorrectly by ignoring system decor.

Test: Apps in split screen now have the background when draging.
Change-Id: I3e310390fcc840d9ae869ed58243a9f8ff5d1337