meta-agl-flutter: update for meta-agl-core changes

meta-agl-flutter: update for meta-agl-core changes

Changes:
- Pull in image .bb files instead of .inc files.
- Use agl-image-compositor as the base image for agl-image-flutter.
  instead of the appending done to agl-image-weston and its
  packagegroup previously.
- Rework flutter-gallery systemd unit to work again, and update our
  bbappend to install it as disabled as an option for starting up
  flutter-gallery in agl-image-flutter.  It seems useful to keep it
  present to serve as an example of starting Flutter apps via
  systemd and using the agl-flutter-env package.
- Some minor documentation updates in README.md.
- Replace some :append usage with += to avoid creating problems
  for downstream users.

Bug-AGL: SPEC-4813

Change-Id: I8a61d6365ce69ac50b88abc7bc451486b3e1b339
Signed-off-by: Scott Murray <scott.murray@konsulko.com>