Update Flutter demo images

Update Flutter demo images

Changes:
- Split demo image feature definitions into an include file to
  allow reusing them, and pull into both the Qt and Flutter demo
  platform images.
- Remove packagegroup-agl-demo-platform dependencies from feature
  packagegroups so they will not break the Flutter images.
- Tweak Flutter cluster demo image to have KUKSA.val related
  package omission be controlled by the "agl-demo-preload"
  feature.
- Add the kuksa-vss-init package to the IVI services packagegroup
  so that the signal initialization expected by the Flutter cluster
  dashboard will always be done.  This covers the usecase of using
  the Flutter cluster with KUKSA.val on the IVI board.
- Add missing "agl-demo-preload" tweaks from the Qt demo platform
  image to the Flutter one.
- Tweak things to leave out the Weston terminal app from images
  build with the "agl-demo-preload" feature.
- Remove flutter-navigation and flutter-mediaplayer from the
  Flutter demo image for now, and add the messaging, phone, and
  radio Qt apps to fill out the list of apps for the demo.

Bug-AGL: SPEC-4636, SPEC-4637, SPEC-4639

Change-Id: I9cde89278f0191886152f765d4fb1a893c0ea2c1
Signed-off-by: Scott Murray <scott.murray@konsulko.com>