Fix smart & default subtitle selection logic (#579)

## Description
Fixes the logic for subtitle selection when using `Smart` or `Default`
server modes. Also takes into account unknown languages.

This is heavily inspired by [the server selection
logic](https://github.com/jellyfin/jellyfin/blob/release-10.11.z/Emby.Server.Implementations/Library/MediaStreamSelector.cs).
For better or for worse, Wholphin implements this client side.

### Related issues
Fixes #570

app/src/main/java/com/github/damontecres/wholphin/data/ItemPlaybackRepository.kt

@@ -61,7 +61,7 @@ class ItemPlaybackRepository
                     )
                 val subtitleStream =
                     streamChoiceService.chooseSubtitleStream(
-                        audioStream = audioStream,
+                        audioStreamLang = audioStream?.language,
                         candidates =
                             source.mediaStreams
                                 ?.filter { it.type == MediaStreamType.SUBTITLE }