Fix MatroskaExtractor to use blockDurationUs not durationUs

This typo was introduced in https://github.com/google/ExoPlayer/commit/ddb70d96ad99f07fe10f53a76ce3262fe625be70
when migrating a static method with parameter `durationUs` to an
instance method where the correct field to use was `blockDurationUs`
(but `durationUs` also exists).

The test that catches this was only added in https://github.com/google/ExoPlayer/commit/45013ece1e3fe054ff8960355a89559241eeb288 (and
therefore configured with the wrong expected output data).

issue:#6833
PiperOrigin-RevId: 288274197

RELEASENOTES.md

@@ -23,6 +23,8 @@
 * WAV:
   * Support IMA ADPCM encoded data.
   * Improve support for G.711 A-law and mu-law encoded data.
+* Fix MKV subtitles to disappear when intended instead of lasting until the
+  next cue ([#6833](https://github.com/google/ExoPlayer/issues/6833)).
 
 ### 2.11.1 (2019-12-20) ###
 

library/core/src/main/java/com/google/android/exoplayer2/extractor/mkv/MatroskaExtractor.java

@@ -1248,10 +1248,10 @@ public class MatroskaExtractor implements Extractor {
       if (CODEC_ID_SUBRIP.equals(track.codecId) || CODEC_ID_ASS.equals(track.codecId)) {
         if (blockSampleCount > 1) {
           Log.w(TAG, "Skipping subtitle sample in laced block.");
-        } else if (durationUs == C.TIME_UNSET) {
+        } else if (blockDurationUs == C.TIME_UNSET) {
           Log.w(TAG, "Skipping subtitle sample with no duration.");
         } else {
-          setSubtitleEndTime(track.codecId, durationUs, subtitleSample.data);
+          setSubtitleEndTime(track.codecId, blockDurationUs, subtitleSample.data);
           // Note: If we ever want to support DRM protected subtitles then we'll need to output the
           // appropriate encryption data here.
           track.output.sampleData(subtitleSample, subtitleSample.limit());