Split shade: fix lockscreen drag down happening on the wrong side

The issue was caused by the horizontal offset of the notifications
container on split shade being "counted" twice.

On handheld this was a non-issue, since the offset is non existent.

Fixes: 264575009
Test: Manually on device. See video.
Change-Id: I04eed5f608bf6f8c65f8a22385e655315ff85f23