Fix starting playback from t!=0

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118032818

library/src/main/java/com/google/android/exoplayer/extractor/ExtractorSampleSource.java

@@ -416,7 +416,7 @@ public final class ExtractorSampleSource implements SampleSource, ExtractorOutpu
         clearState();
         allocator.trim(0);
       }
-    } else if (!isFirstTrackSelection && newTracksSelected) {
+    } else if (isFirstTrackSelection ? positionUs != 0 : newTracksSelected) {
       seekToInternal(positionUs);
     }
   }