Rework cluster demo configuration

Rework cluster demo configuration

Changes:
- Rename cluster-dashboard-demo-config to cluster-demo-config since
  the installed AGL.conf will also be used for receiver configuration
  now.
- Add a Flutter cluster version of AGL.conf that configures the
  receiver window geometry as required.  The alternative mechanism
  is used to generate separate packages with the different AGL.conf
  files.
- Update the package names installed by agl-demo-preload in the Qt
  and Flutter cluster demo images.

It may make sense to do further rework in the dashboard and receiver
apps post-CES to split things into separate configuration files, but
this seems the most straightforward way to do things for now.

Bug-AGL: SPEC-4640

Change-Id: If8a65228c75f28a53ef32a49be90f75a85d60a16
Signed-off-by: Scott Murray <scott.murray@konsulko.com>