Use artist ID for primary image in constructors

When using the constructor with an Album or Song, default the image to
the artist ID so the artist image always shows up when set.
Fixes #203 and has no side effects as far as I can see.

app/src/main/java/com/dkanada/gramophone/model/Artist.java

@@ -46,11 +46,13 @@ public class Artist implements Parcelable {
     public Artist(Album album) {
         this.id = album.artistId;
         this.name = album.artistName;
+        this.primary = this.id;
     }
 
     public Artist(Song song) {
         this.id = song.artistId;
         this.name = song.artistName;
+        this.primary = this.id;
     }
 
     public Artist() {