Fix DefaultDashChunkSource.updateLiveEdgeTimeUs

Very subtle, but lastAvailableSegmentNum is shifted by
RepresentationHolder.segmentNumShift. When accessing the
index directly it's necessary to unshift. The easiest way
to do this is to call through the holder, which does this
for you.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=169239928

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DefaultDashChunkSource.java

@@ -317,15 +317,8 @@ public class DefaultDashChunkSource implements DashChunkSource {
 
   private void updateLiveEdgeTimeUs(RepresentationHolder representationHolder,
       int lastAvailableSegmentNum) {
-    if (manifest.dynamic) {
-      DashSegmentIndex segmentIndex = representationHolder.representation.getIndex();
-      long lastSegmentDurationUs = segmentIndex.getDurationUs(lastAvailableSegmentNum,
-          manifest.getPeriodDurationUs(periodIndex));
-      liveEdgeTimeUs = segmentIndex.getTimeUs(lastAvailableSegmentNum)
-          + lastSegmentDurationUs;
-    } else {
-      liveEdgeTimeUs = C.TIME_UNSET;
-    }
+    liveEdgeTimeUs = manifest.dynamic
+        ? representationHolder.getSegmentEndTimeUs(lastAvailableSegmentNum) : C.TIME_UNSET;
   }
 
   private long getNowUnixTimeUs() {