desktop: Install a black background for REMOTE roles

desktop: Install a black background for REMOTE roles

A while back we added the ability to install a black curtain if there no
applications running. This way it provides a visual cue and it signifies
that the applications is no longer running.

While the idea was to install a remote black background for the REMOTE
role, it turns out this would only happened if we were using the Waltham
output type, which is excluding the REMOTE role. Adds an explicit check
for Waltham to still allow for cases where just the REMOTE role is used.

Bug-AGL: SPEC-4520
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I42951faa7cfb6932fa0e42e75f79245a82181f06