applaunchd: update Flutter app service template

applaunchd: update Flutter app service template

Update Flutter app template service unit for the new embedder
command line option syntax.  Also remove the now stale default
Flutter version / runtime environment variable definitions, as
we rely on the definitions from the agl-flutter-env package
being present, anyways.

Bug-AGL: SPEC-5326

Change-Id: Ia5352cb2c3ca0490d0e8a474957507c440785ec6
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/30699
ci-image-build: Jenkins Job builder account
Tested-by: Jenkins Job builder account
Reviewed-by: Marius Vlad <marius.vlad@collabora.com>
ci-image-boot-test: Jenkins Job builder account