Set HlsSampleStreamWrapper.trackType for audio-only playlists

For audio-only playlists, when formats are communicated to the app with
AnalyticsListener.onDownstreamFormatChanged(), the passed MediaLoadData
do not indicate this is an audio track and therefore the
PlaybackStatsListener cannot derive audio format-related information.

This change sets the main SampleStreamWrappers track type to AUDIO, if
the master playlist contains only audio variants.

Issue: #9175

#minor-release

PiperOrigin-RevId: 388676060

RELEASENOTES.md

@@ -144,6 +144,12 @@
  * Deprecate `setControlDispatcher` in `MediaSessionConnector`. The
  `ControlDispatcher` parameter has also been deprecated in all
  `MediaSessionConnector` listener methods.
+* HLS:
+ * Report audio track type in
+ `AnalyticsListener.onDownstreamFormatChanged()` for audio-only
+ playlists, so that the `PlaybackStatsListener` can derive audio
+ format-related information.
+ ([#9175](https:/google/ExoPlayer/issues/9175)).
 
 ### 2.14.2 (2021-07-20)
 

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaPeriod.java

@@ -628,9 +628,13 @@ private void buildAndPrepareMainSampleStreamWrapper(
  numberOfAudioCodecs <= 1
  && numberOfVideoCodecs <= 1
  && numberOfAudioCodecs + numberOfVideoCodecs > 0;
+ int trackType =
+ !useVideoVariantsOnly && numberOfAudioCodecs > 0
+ ? C.TRACK_TYPE_AUDIO
+ : C.TRACK_TYPE_DEFAULT;
  HlsSampleStreamWrapper sampleStreamWrapper =
  buildSampleStreamWrapper(
- C.TRACK_TYPE_DEFAULT,
+ trackType,
  selectedPlaylistUrls,
  selectedPlaylistFormats,
  masterPlaylist.muxedAudioFormat,