Correctly propagate format identifier for CEA-608 in HLS

Issue: #3033

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161512537

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

@@ -51,9 +51,10 @@ import java.util.List;
       Assertions.checkArgument(MimeTypes.APPLICATION_CEA608.equals(channelMimeType)
           || MimeTypes.APPLICATION_CEA708.equals(channelMimeType),
           "Invalid closed caption mime type provided: " + channelMimeType);
-      output.format(Format.createTextSampleFormat(idGenerator.getFormatId(), channelMimeType, null,
-          Format.NO_VALUE, channelFormat.selectionFlags, channelFormat.language,
-          channelFormat.accessibilityChannel, null));
+      String formatId = channelFormat.id != null ? channelFormat.id : idGenerator.getFormatId();
+      output.format(Format.createTextSampleFormat(formatId, channelMimeType, null, Format.NO_VALUE,
+          channelFormat.selectionFlags, channelFormat.language, channelFormat.accessibilityChannel,
+          null));
       outputs[i] = output;
     }
   }