Allow regressing media sequence numbers in HLS media playlists

This is techically not allowed by the spec[1] but might still occur in
certain scenarios. New playlists with older media sequence numbers are
ignored.

[1]: HLS draft version 20, section-6.2.1

Issue:#2059

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139564889

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

@@ -311,6 +311,10 @@ public final class HlsPlaylistTracker implements Loader.Callback<ParsingLoadable
       // Playlist has not changed.
       // Playlist has not changed.
       return oldPlaylist;
       return oldPlaylist;
     }
     }
+    if (mediaSequenceOffset < 0) {
+      // Playlist has changed but media sequence has regressed.
+      return oldPlaylist;
+    }
     if (mediaSequenceOffset <= oldPlaylistSize) {
     if (mediaSequenceOffset <= oldPlaylistSize) {
       // We can extrapolate the start time of new segments from the segments of the old snapshot.
       // We can extrapolate the start time of new segments from the segments of the old snapshot.
       ArrayList<HlsMediaPlaylist.Segment> newSegments = new ArrayList<>(newPlaylistSize);
       ArrayList<HlsMediaPlaylist.Segment> newSegments = new ArrayList<>(newPlaylistSize);