Run applaunchd under a separate user

Since applaunchd needs to start/stop systemd units, the user is granted elevated systemd unit-management permissions via PolKit policy. If applaunchd and all the apps run under the same agl-driver user, all the apps have these elevated systemd permissions too. Separating them into different users allows removing elevated systemd unit-management permission from individual apps, but leaving such permission for applaunchd, which enhances overall security of the system.