Fix seeking in live HLS streams

Issue:#4187

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194829857

RELEASENOTES.md

@@ -81,6 +81,8 @@
     ([#4016](https://github.com/google/ExoPlayer/issues/4016)).
   * Add support for multiple #EXT-X-MAP tags in a media playlist
     ([#4164](https://github.com/google/ExoPlayer/issues/4182)).
+  * Fix seeking in live streams
+    ([#4187](https://github.com/google/ExoPlayer/issues/4187)).
 * Fix ClearKey decryption error if the key contains a forward slash
   ([#4075](https://github.com/google/ExoPlayer/issues/4075)).
 * Fix crash when switching surface on Huawei P9 Lite

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsChunkSource.java

@@ -261,10 +261,12 @@ import java.util.List;
         // If the playlist is too old to contain the chunk, we need to refresh it.
         chunkMediaSequence = mediaPlaylist.mediaSequence + mediaPlaylist.segments.size();
       } else {
+        // The playlist start time is subtracted from the target position because the segment start
+        // times are relative to the start of the playlist, but the target position is not.
         chunkMediaSequence =
             Util.binarySearchFloor(
                     mediaPlaylist.segments,
-                    targetPositionUs,
+                    /* value= */ targetPositionUs - mediaPlaylist.startTimeUs,
                     /* inclusive= */ true,
                     /* stayInBounds= */ !playlistTracker.isLive() || previous == null)
                 + mediaPlaylist.mediaSequence;