Allow overridable empty closed caption format list in DefaultTsPayloadReaderFactory

Issue:#4451

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202923630

library/core/src/main/java/com/google/android/exoplayer2/extractor/ts/DefaultTsPayloadReaderFactory.java

@@ -61,7 +61,10 @@ public final class DefaultTsPayloadReaderFactory implements TsPayloadReader.Fact
    *     readers.
    */
   public DefaultTsPayloadReaderFactory(@Flags int flags) {
-    this(flags, Collections.<Format>emptyList());
+    this(
+        flags,
+        Collections.singletonList(
+            Format.createTextSampleFormat(null, MimeTypes.APPLICATION_CEA608, 0, null)));
   }
 
   /**
@@ -76,10 +79,6 @@ public final class DefaultTsPayloadReaderFactory implements TsPayloadReader.Fact
    */
   public DefaultTsPayloadReaderFactory(@Flags int flags, List<Format> closedCaptionFormats) {
     this.flags = flags;
-    if (!isSet(FLAG_OVERRIDE_CAPTION_DESCRIPTORS) && closedCaptionFormats.isEmpty()) {
-      closedCaptionFormats = Collections.singletonList(Format.createTextSampleFormat(null,
-          MimeTypes.APPLICATION_CEA608, 0, null));
-    }
     this.closedCaptionFormats = closedCaptionFormats;
   }
 

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

@@ -93,7 +93,11 @@ public final class DefaultHlsExtractorFactory implements HlsExtractorFactory {
         // The playlist declares closed caption renditions, we should ignore descriptors.
         esReaderFactoryFlags |= DefaultTsPayloadReaderFactory.FLAG_OVERRIDE_CAPTION_DESCRIPTORS;
       } else {
-        muxedCaptionFormats = Collections.emptyList();
+        // The playlist does not provide any closed caption information. We preemptively declare a
+        // closed caption track on channel 0.
+        muxedCaptionFormats =
+            Collections.singletonList(
+                Format.createTextSampleFormat(null, MimeTypes.APPLICATION_CEA608, 0, null));
       }
       String codecs = format.codecs;
       if (!TextUtils.isEmpty(codecs)) {