libs/wlgen/workload: fix background execution for Android targets
Created by: JaviMerino
Commit c83009d8 ("libs/wlgen/workload: fix background execution when CGroups are in use") changed the command for a Workload that runs in the background from target.background() to target.kick_off(). Sadly, target.kick_off() is broken for Android targets, see ARM-software/devlib#55.
As c83009d8 only talked about problems with cgroups and target.background(), probably due to interactions with cgroup_run_into.sh. As a compromise, run target.kick_off() for cgroup
and target.background() if cgroup is not set.