Query the "active-state" property of units in the "job-removed" handler

Query the "active-state" property of units in the "job-removed" handler

Our previous attempt to fix the TargetStartupMonitor failed because when
a target is stopped, systemd drops the unit from the system bus and the
proxies we created in TargetStartupMonitor were no longer associated
with these unit objects that previously existed on the bus. When such a
target is started again, the proxy will no longer receive property
change signals from systemd.

This commit tries to implement the TargetStartupMonitor in yet another
way, which is to listen to systemd's "job-removed" signal again. In
response to receiving "job-removed" signals, we now create a SystemdUnit
for the affected unit and query that for its current "active-state"
value. Perhaps this will work.