Add missing divider height before notification shelf

The mismatch between computed height and actual height required
caused the notification right before the shelf to be
slightly clipped, pushing its icon slightly into the shelf
and messing up icon overflow calculations.

Bug: 227361193
Test: manually inspect shelf icon placement
	=> see that notification before shelf
	   is no longer incorrectly clipped
Change-Id: Icb787f5b141b7df3b3ae801a2d89925d99924034