meta-agl-flutter: Initial Kirkstone updates

meta-agl-flutter: Initial Kirkstone updates

Changes:
- Unpin Flutter SDK tag and ivi-homescreen SRCREV to track the
  upstream development against kirkstone for now.  This at least
  gets us to building an image.  It seems likely that when the
  Needlefish release happens these may need to be pinned again
  for reproducibility.
- Added a remove of the "gstreamer" option from the ivi-homescreen
  PACKAGECONFIG in the local bbappend.  This avoids needing to
  enable "commercial" licenses to allow ffmpeg to be pulled in.
  This will likely need further investigation around just what
  pieces are needed for upstream, as currently Flutter video
  playback is tied to it.  It may be that an AGL feature to
  enable commercial licenses specifically for this becomes
  necessary...

Note that while the above does allow building agl-image-flutter,
ivi-homescreen currently fails to start, so further investigation
is required.

Bug-AGL: SPEC-3819

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I40f0619a161c4ebe1f37dc840caf9bbd6bc95947