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
