Add a gray debug line to NSSL to show the keyguard padding.

Bug: 190630151
Test: Disable the keyguard max; add 5 normal notifications on a sargo, see them all draw right up to the gray line.  Replace 1 with a conversation (adds a few dp padding) and see the shelf appear.  Soo.. seems consistent.
Change-Id: Id5f93b7151dd0b1698ceebda9a01c592ba54bb2f