notification stack scroller doesn't need a11y focus

Only its contents need to get focus. This was causing the empty space
on the lock screen to get a11y focus with the description "no label"

Test: manual (large clock - tap in empty space, no focus is given)
Fixes: 195614563
Change-Id: If8b8d069ef111a8df1bc659e25dd5c01a79528a2