applaunchd unable to start applications on reference hardware

As reported by jsmoeller on one of this morning's calls, and seen locally by myself, with latest master and the Pike M1 candidate trying to start applications from the homescreen in the agl-ivi-demo-platform-flutter image fails on the reference hardware with an applaunchd error visible in the logs.  With some investigation, it looks like something in recent changes (perhaps https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29016 in combination with the systemd user session removal) is tickling a race condition with respect to applaunchd and polkitd.  We have never encoded an explicit dependency, but applaunchd effectively needs polkitd to be up for its systemd DBus API requests to work.  From digging around with systemctl and looking in /usr/lib/systemd/system, I cannot find an inherent dependency that would guarantee this, so it's likely we do need to make it explicit.