Fix negative start time values

Issue:#2495

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148348663

library/src/androidTest/java/com/google/android/exoplayer2/source/hls/playlist/HlsMediaPlaylistParserTest.java

@@ -36,6 +36,7 @@ public class HlsMediaPlaylistParserTest extends TestCase {
     String playlistString = "#EXTM3U\n"
         + "#EXT-X-VERSION:3\n"
         + "#EXT-X-PLAYLIST-TYPE:VOD\n"
+        + "#EXT-X-START:TIME-OFFSET=-25"
         + "#EXT-X-TARGETDURATION:8\n"
         + "#EXT-X-MEDIA-SEQUENCE:2679\n"
         + "#EXT-X-DISCONTINUITY-SEQUENCE:4\n"
@@ -73,6 +74,7 @@ public class HlsMediaPlaylistParserTest extends TestCase {
 
       HlsMediaPlaylist mediaPlaylist = (HlsMediaPlaylist) playlist;
       assertEquals(HlsMediaPlaylist.PLAYLIST_TYPE_VOD, mediaPlaylist.playlistType);
+      assertEquals(mediaPlaylist.durationUs - 25000000, mediaPlaylist.startOffsetUs);
 
       assertEquals(2679, mediaPlaylist.mediaSequence);
       assertEquals(3, mediaPlaylist.version);

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

@@ -81,7 +81,7 @@ public final class HlsPlaylistParser implements ParsingLoadable.Parser<HlsPlayli
       + ":(\\d+)\\b");
   private static final Pattern REGEX_MEDIA_DURATION = Pattern.compile(TAG_MEDIA_DURATION
       + ":([\\d\\.]+)\\b");
-  private static final Pattern REGEX_TIME_OFFSET = Pattern.compile("TIME-OFFSET=([\\d\\.]+)\\b");
+  private static final Pattern REGEX_TIME_OFFSET = Pattern.compile("TIME-OFFSET=(-?[\\d\\.]+)\\b");
   private static final Pattern REGEX_BYTERANGE = Pattern.compile(TAG_BYTERANGE
       + ":(\\d+(?:@\\d+)?)\\b");
   private static final Pattern REGEX_ATTR_BYTERANGE =