meta-agl-flutter: fix sample app installation

meta-agl-flutter: fix sample app installation

Fix TOML configuration file installation in the recipe for
the flutter-samples-material-3-demo sample app used in
agl-image-flutter.  Also tweak command-line in its systemd
unit to match embedder expectations.

Bug-AGL: SPEC-5362

Change-Id: I048b937bf5622504634f53c979b5442f3c325645
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/30838
ci-image-build: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account