sleep,home: always initialize UnitFreezer if used

sleep,home: always initialize UnitFreezer if used

Previously, unit_freezer_new_freeze() would only return
UnitFreezer object if FreezeUnit() succeeds. This is not
ideal though, as a failed bus call doesn't mean the action
actually failed. E.g. a timeout might occur because pid1
is waiting for cgroup event from kernel, while the bus call
timeout was exceeded (#33269). In such a case, ThawUnit()
will never be called, resulting in frozen units remain that
way after resuming from sleep.

Therefore, let's get rid of unit_freezer_new_freeze(),
and make sure as long as unit freezer is involved, we'll
call ThawUnit() when we're done. This should make things
a lot more robust.