Implement subtitle and audio track language preferences

This is an external contribution that allows the user to setup a preferred language for the audio and subtitle tracks.

I relies on string comparison:

• the preference is an EditTextPreference that allows the user to set "fr" for example
• when a track is added during video playback, it will switch to the first track containing this string
• this commit respects the external media metadata track selection (if a previous playback has been made using a different track, this track is restored).

I opened this MR to start the discussion about how this approach is relevant and to allow improving the code if needed.