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://github.com/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,