agl-persistent-storage-api: run as agl-driver

agl-persistent-storage-api: run as agl-driver

Tweak the agl-persistent-storage-api systemd unit to run the daemon
as the agl-driver user instead of root.  Another non-root user would
also be an option, but since there's no authentication scheme for the
API, running it as the same user as the applications seems reasonable
for now.

Bug-AGL: SPEC-5250

Change-Id: I57c9f8c108dd78b1fd6e5b011ed39e81946ab0bc
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/30317
ci-image-build: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Naoto YAMAGUCHI <naoto.yamaguchi@aisin.co.jp>
Tested-by: Jenkins Job builder account