Avoid adding transparency to Shelf Background View

NotificationBackgroundView is a super class for both notification rows,
and the NotificationShelfBackgroundView. The notificationRowTransparency
feature adds transparency to notification rows, which caused a
side-effect that made the shelf transparent or semi-transparent.

Bug: 420181235
Flag: EXEMPT bugfix
Test: manual, open shade, close shade without swiping to the end, check
      shelf on lockscreen.
Change-Id: Id780e45d730217be34462722de831f5b7155d11d