nixos/autorandr: make default target in systemd service configurable

The `.service` file defining the `systemd` unit for `autorandr.service`
which is bundled with the package itself uses `--default default` in the
`ExecStart` section. This can be an issue when having multiple layouts
(e.g. `default` as workstation layout I mostly work on and `mobile` when
I go somewhere else).

When the service gets restarted and `--default` can't be applied,
however the current layout can't be detected (e.g. when working with an
unknown beamer) the service silently fails with a message like this:

```
Jun 22 18:44:46 hauptshuhle autorandr[3168]: /nix/store/h83b72ffm68nm8fyjnppljchp456a94r-xrandr-1.5.0/bin/xrandr: ca>
Jun 22 18:44:46 hauptshuhle autorandr[3168]: Failed to apply profile 'default' (line 718):
Jun 22 18:44:46 hauptshuhle autorandr[3168]:   Command failed: /nix/store/h83b72ffm68nm8fyjnppljchp456a94r-xrandr-1.>
```

As discussed in the IRC (see https://botbot.me/freenode/nixos/2018-07-05/?msg=101791455&page=6)
it's a bad long-term solution in terms of maintenance to manually patch
the service file bundled with the derivation, instead the service shall
be configured declaratively. Additionally this makes possible overrides
from the user-space way easier.

The `udev` rule (in `$out/etc/udev/rules.d`) won't' be affected, it
simply runs `systemctl start autorandr.service` when e.g. a new display
is added, so now `udev` communicates with the NixOS systemd unit.