Fix incorrect background bounds set by stackscroll view

Even if the stackscroll view is the same width as notification panel
view, it doesn't guarantee the left position is 0 in window(when nav bar
is on the left side).

Always use location in window to set the left position instead.

Test: runtest systemui
Test: manual 1) change display size to largest 2) rotate to landscape 3)
expand notification panel and check
Fixes: 74988082

Change-Id: I06641c145365e315192e0b2b6c44d075a3016733