Increase vertical overlap between notification shelf and lock icon

Fixes: 214257536

Test: media player/group notification/shelf fits in space available
      with increased overlap between shelf and lock icon

Test: shelf-less stack does not overlap lock icon

Test: NotificationStackSizeCalculatorTest

Test: NotificationPanelViewControllerTest

Change-Id: I718a9d7e81b53277e19e588fbc85502cb2ed6b6c