Make MatroskaExtractor.seek() non-final

Now that this class can be extended, it makes sense for subclasses to be able
to clear state in seek().

PiperOrigin-RevId: 237902276

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

@@ -412,8 +412,9 @@ public class MatroskaExtractor implements Extractor {
     extractorOutput = output;
   }
 
+  @CallSuper
   @Override
-  public final void seek(long position, long timeUs) {
+  public void seek(long position, long timeUs) {
     clusterTimecodeUs = C.TIME_UNSET;
     blockState = BLOCK_STATE_START;
     reader.reset();