BlackBerry: Fix metadata key for "artist"

In order to retrieve the metadata information describing the Artist of a given
album, Qt queries for AlbumArtist, and not Author. Author actually refers to
the person who created the media file (i.e. the encoder in case of a mp3
song), and is also a string list (since more than one author is supported).

Task-number: QTBUG-34985

Change-Id: I30b61f83db07ad2274eb79b28d92d772654220de
Reviewed-by: Tobias Koenig <tobias.koenig.qnx@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>