Enable persistent storage API in Flutter IVI demo

Enable persistent storage API in Flutter IVI demo

Changes:
- Bump flutter-ics-homescreen SRCREV to pick up the storage API
  changes.
- Add a dependency on agl-persistent-storage-api to the homescreen
  systemd unit to ensure it will be started beforehand.
- Add an environment file to be pulled in by the homescreen systemd
  unit that sets SPDLOG_LEVEL to enable debug output with latest
  flutter-auto.  This should help with debugging any future issues.
- Add agl-persistent-storage-api to the base Flutter IVI image.
  This gets it into the final demo image, and keeps the base Flutter
  IVI image (agl-image-ivi-flutter) usable for potentially working
  on the homescreen with the workspace-automation tooling.

Bug-AGL: SPEC-5250

Change-Id: Icd2958a259f8eada8db0bfb99b47ba7a64094fa5
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/30318
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
ci-image-boot-test: Jenkins Job builder account
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account