Only show status bar blocker view if nav bar is translucent.

It's the background drawable of the translucent nav bar that makes the
blocker view necessary (as the background creates a visual 'gap' between
status bar and visual start of the nav bar), so if the nav bar isn't
translucent, there's no need for the view.

Change-Id: I684c8d24abe3d4393ae7db88fe5023aa65836164