Fix / improve format merging

1. Prefer label and language values in the manifest to those in
   the media. This is particularly helpful if the sample format
   contains "und" as the language.
2. Copy label when deriving formats in HlsSampleStreamWrapper
3. When there's only one variant in HlsSampleStreamWrapper, use
   the regular copyWithManifestFormatInfo. This allows more
   information to be retained from the sample format.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=204340008

library/core/src/main/java/com/google/android/exoplayer2/Format.java

@@ -1041,6 +1041,7 @@ public final class Format implements Parcelable {
 
   public Format copyWithContainerInfo(
       @Nullable String id,
+      @Nullable String label,
       @Nullable String sampleMimeType,
       @Nullable String codecs,
       int bitrate,
@@ -1084,15 +1085,21 @@ public final class Format implements Parcelable {
       // No need to copy from ourselves.
       return this;
     }
+
+    // Use manifest value only.
     String id = manifestFormat.id;
+    // Prefer manifest values, but fill in from sample format if missing.
+    String label = manifestFormat.label != null ? manifestFormat.label : this.label;
+    String language = manifestFormat.language != null ? manifestFormat.language : this.language;
+    // Prefer sample format values, but fill in from manifest if missing.
     String codecs = this.codecs == null ? manifestFormat.codecs : this.codecs;
     int bitrate = this.bitrate == NO_VALUE ? manifestFormat.bitrate : this.bitrate;
     float frameRate = this.frameRate == NO_VALUE ? manifestFormat.frameRate : this.frameRate;
-    @C.SelectionFlags int selectionFlags = this.selectionFlags |  manifestFormat.selectionFlags;
-    String language = this.language == null ? manifestFormat.language : this.language;
-    String label = this.label == null ? manifestFormat.label : this.label;
+    // Merge manifest and sample format values.
+    @C.SelectionFlags int selectionFlags = this.selectionFlags | manifestFormat.selectionFlags;
     DrmInitData drmInitData =
         DrmInitData.createSessionCreationData(manifestFormat.drmInitData, this.drmInitData);
+
     return new Format(
         id,
         label,

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

@@ -948,8 +948,12 @@ import java.util.List;
       Format sampleFormat = sampleQueues[i].getUpstreamFormat();
       if (i == primaryExtractorTrackIndex) {
         Format[] formats = new Format[chunkSourceTrackCount];
-        for (int j = 0; j < chunkSourceTrackCount; j++) {
-          formats[j] = deriveFormat(chunkSourceTrackGroup.getFormat(j), sampleFormat, true);
+        if (chunkSourceTrackCount == 1) {
+          formats[0] = sampleFormat.copyWithManifestFormatInfo(chunkSourceTrackGroup.getFormat(0));
+        } else {
+          for (int j = 0; j < chunkSourceTrackCount; j++) {
+            formats[j] = deriveFormat(chunkSourceTrackGroup.getFormat(j), sampleFormat, true);
+          }
         }
         trackGroups[i] = new TrackGroup(formats);
         primaryTrackGroupIndex = i;
@@ -997,7 +1001,8 @@ import java.util.List;
   }
 
   /**
-   * Derives a track format using master playlist and sample format information.
+   * Derives a track sample format from the corresponding format in the master playlist, and a
+   * sample format that may have been obtained from a chunk belonging to a different track.
    *
    * @param playlistFormat The format information obtained from the master playlist.
    * @param sampleFormat The format information obtained from the samples.
@@ -1019,6 +1024,7 @@ import java.util.List;
     }
     return sampleFormat.copyWithContainerInfo(
         playlistFormat.id,
+        playlistFormat.label,
         mimeType,
         codecs,
         bitrate,

library/ui/src/main/java/com/google/android/exoplayer2/ui/DefaultTrackNameProvider.java

@@ -43,11 +43,11 @@ public class DefaultTrackNameProvider implements TrackNameProvider {
     } else if (trackType == C.TRACK_TYPE_AUDIO) {
       trackName =
           joinWithSeparator(
-              buildLanguageString(format),
+              buildLabelString(format),
               buildAudioChannelString(format),
               buildBitrateString(format));
     } else {
-      trackName = buildLanguageString(format);
+      trackName = buildLabelString(format);
     }
     return trackName.length() == 0 ? resources.getString(R.string.exo_track_unknown) : trackName;
   }
@@ -87,7 +87,11 @@ public class DefaultTrackNameProvider implements TrackNameProvider {
     }
   }
 
-  private String buildLanguageString(Format format) {
+  private String buildLabelString(Format format) {
+    if (!TextUtils.isEmpty(format.label)) {
+      return format.label;
+    }
+    // Fall back to using the language.
     String language = format.language;
     return TextUtils.isEmpty(language) || C.LANGUAGE_UNDETERMINED.equals(language)
         ? ""