meta-agl-flutter: rework agl-image-flutter

meta-agl-flutter: rework agl-image-flutter

Rework agl-image-flutter to serve more as an example of including
a Flutter application into a production image, while also keeping
it being somewhat of a bit of a build smoketest for meta-flutter
integration.

Changes:
- Switch agl-image-flutter over to using the release version of
  the Flutter engine, and dropping the SDK, dev, and test packages.
- Re-enable having the gallery application get auto-started.
- Add a "-debug" version of the image that keeps most of the dev
  bits from the previous agl-image-flutter, and bakes in stuff
  like package management and debug tools.  This keeps an example
  of how such an image can be built, and will still work in the
  standalone case when agl-devel is not used.

Bug-AGL: SPEC-5182

Change-Id: I8e59fe71f8739ae67dd9f8c743a7024fd4f52b84
Signed-off-by: Scott Murray <scott.murray@konsulko.com>