fix: Populate track's spatialAudio property (shaka-project#4291)

Fixes issue shaka-project#4290 where `spatialAudio` flag is not being set on variant tracks.

Co-authored-by: Dan Sparacio <daniel.sparacio@cbsinteractive.com>

lib/util/stream_utils.js

@@ -1015,6 +1015,7 @@ shaka.util.StreamUtils = class {
       track.channelsCount = audio.channelsCount;
       track.audioSamplingRate = audio.audioSamplingRate;
       track.audioBandwidth = audio.bandwidth || null;
+      track.spatialAudio = audio.spatialAudio;
       track.label = audio.label;
       track.audioRoles = audio.roles;
     }