Treat seeks in HLS live to be at t=0, as in ExtractorSampleSource.

This makes sense until we need to support seeking in the live window.

Issue: #676
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112402026

library/src/main/java/com/google/android/exoplayer/hls/HlsChunkSource.java

@@ -260,6 +260,17 @@ public class HlsChunkSource implements HlsTrackSelector.Output {
     return fatalError == null;
   }
 
+  /**
+   * Returns whether this is a live playback.
+   * <p>
+   * This method should only be called after the source has been prepared.
+   *
+   * @return True if this is a live playback. False otherwise.
+   */
+  public boolean isLive() {
+    return live;
+  }
+
   /**
    * Returns the duration of the source, or {@link C#UNKNOWN_TIME_US} if the duration is unknown.
    * <p>

library/src/main/java/com/google/android/exoplayer/hls/HlsSampleSource.java

@@ -206,6 +206,8 @@ public final class HlsSampleSource implements SampleSource, SampleSourceReader,
       loadControl.register(this, bufferSizeContribution);
       loadControlRegistered = true;
     }
+    // Treat enabling of a live stream as occurring at t=0 in both of the blocks below.
+    positionUs = chunkSource.isLive() ? 0 : positionUs;
     int chunkSourceTrack = chunkSourceTrackIndices[track];
     if (chunkSourceTrack != -1 && chunkSourceTrack != chunkSource.getSelectedTrackIndex()) {
       // This is a primary track whose corresponding chunk source track is different to the one
@@ -359,6 +361,8 @@ public final class HlsSampleSource implements SampleSource, SampleSourceReader,
   public void seekToUs(long positionUs) {
     Assertions.checkState(prepared);
     Assertions.checkState(enabledTrackCount > 0);
+    // Treat all seeks into live streams as being to t=0.
+    positionUs = chunkSource.isLive() ? 0 : positionUs;
 
     // Ignore seeks to the current position.
     long currentPositionUs = isPendingReset() ? pendingResetPositionUs : downstreamPositionUs;