layout: Remove black surface when trying to use migrate views to other

layout: Remove black surface when trying to use migrate views to other
outputs

We install a black surface in the fullscreen layer for each output, and
we need to remove it before adding the new surfaces/views. We did that
aleady for 'remote' type of surfaces when categorizing the type, but
here we use the configuration file to determine the output, but we were
missing the black surface for that output. This verifies that the black
surface for that output is in use, and removes the black surface if so.

Bug-AGL: SPEC-3280

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I3c4ae90ee0983bc424c932715d9a1ff082117bef