Handle meta-flutter update

Changes:
- Rename flutter-gallery bbappend to handle switch to BBCLASSEXTEND
  scheme in the upstream recipe.
- Update the Flutter embedder package name in the Flutter demo image
  package group to handle upstream change.

Bug-AGL: SPEC-4485

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I3d5cf4e6f38b766c5dc65979cb53b6674af2b03a