applaunchd: update Flutter app systemd template unit

applaunchd: update Flutter app systemd template unit

Applications built with newer meta-flutter package their bundles in
a directory hierarchy containing the Flutter SDK version and
multiple runtime flavors (release, debug, profile).  Rework our
applaunchd systemd template unit to work with that.  FLUTTER_VERSION
and FLUTTER_RUNTIME environment variables are used in the ExecStart
command line, with working defaults defined in the unit.  The unit
also includes sourcing a global /etc/default/flutter environment
file as well as a per-application one to give us hooki for setting
the version based on the value of FLUTTER_SDK_TAG in meta-agl-flutter.

Bug-AGL: SPEC-4819

Change-Id: I4879760bfd9c01f29daf25d6cd4a3c70d07429fd
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28979
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>