convert: `embed` setting not respected on import (in `auto` mode)

beets git version

```
....
plugins: convert replaygain lastgenre fetchart info

fetchart:
auto: yes
maxwidth: 300
cautious: yes
match:
strong_rec_thresh: 0.30
max_rec:
missing_tracks: strong

import:
art: yes
languages: es en

paths:
default: $albumartist/$album/$track - $title
singleton: $artist - $title
comp: Varios/$album/$track - $title

lastgenre:
canonical: yes

convert:
auto: yes
tmpdir: /tmp
format: mp3
max_bitrate: 250
embed: yes
quiet: yes
......
```

After `beet import` an album, it downloads the album art and transcode the files but it doesn't embed the art:

```
$ beet info -i 'art' Música/Avicii/Stories/01\ -\ Waiting\ for\ Love.mp3
art: False
```