logind: fix getting property OnExternalPower via D-Bus

logind: fix getting property OnExternalPower via D-Bus

The BUS_DEFINE_PROPERTY_GET_GLOBAL macro requires a value as third
argument, so we need to call manager_is_on_external_power(). Otherwise
the function pointer is interpreted as a boolean and always returns
true:

```
$ busctl get-property org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager OnExternalPower
b true
$ /lib/systemd/systemd-ac-power  --verbose
no
```

Thanks: Helmut Grohne <helmut@subdivi.de>
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1021644