Sanity check current position <= written frame position

This avoids spurious position reports following an underrun.

Github: #1874

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170344399

library/core/src/main/java/com/google/android/exoplayer2/audio/DefaultAudioSink.java

@@ -332,6 +332,7 @@ public final class DefaultAudioSink implements AudioSink {
       }
     }
 
+    positionUs = Math.min(positionUs, framesToDurationUs(getWrittenFrames()));
     return startMediaTimeUs + applySpeedup(positionUs);
   }