Optimize HLS seeking.

I think the concept of a sparse track might need formalizing
in Format at some point. We should probably do a similar thing
with sparse tracks in ExtractorSampleSource as well. WDYT?

Issue: #551
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120530195

library/src/main/java/com/google/android/exoplayer/extractor/ExtractorSampleSource.java

@@ -567,8 +567,10 @@ public final class ExtractorSampleSource implements SampleSource, ExtractorOutpu
     // If we're not pending a reset, see if we can seek within the sample queues.
     boolean seekInsideBuffer = !isPendingReset();
     for (int i = 0; seekInsideBuffer && i < sampleQueues.length; i++) {
+      if (trackEnabledStates[i]) {
         seekInsideBuffer = sampleQueues[i].skipToKeyframeBefore(positionUs);
       }
+    }
     // If we failed to seek within the sample queues, we need to restart.
     if (!seekInsideBuffer) {
       restartFrom(positionUs);

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

@@ -532,9 +532,27 @@ public final class HlsSampleSource implements SampleSource, Loader.Callback {
     lastSeekPositionUs = positionUs;
     downstreamPositionUs = positionUs;
     Arrays.fill(pendingResets, true);
+    boolean seekInsideBuffer = !isPendingReset();
+    // TODO[REFACTOR]: This will nearly always fail to seek inside all buffers due to sparse tracks
+    // such as ID3 (probably EIA608 too). We need a way to not care if we can't seek to the keyframe
+    // before for such tracks. For ID3 we probably explicitly don't want the keyframe before, even
+    // if we do have it, since it might be quite a long way behind the seek position. We probably
+    // only want to output ID3 buffers whose timestamps are greater than or equal to positionUs.
+    for (int i = 0; seekInsideBuffer && i < sampleQueues.length; i++) {
+      if (groupEnabledStates[i]) {
+        seekInsideBuffer = sampleQueues[i].skipToKeyframeBefore(positionUs);
+      }
+    }
+    if (seekInsideBuffer) {
+      while (mediaChunks.size() > 1 && mediaChunks.get(1).startTimeUs <= positionUs) {
+        mediaChunks.removeFirst();
+      }
+    } else {
+      // If we failed to seek within the sample queues, we need to restart.
       chunkSource.seek();
       restartFrom(positionUs);
     }
+  }
 
   private void discardSamplesForDisabledTracks() {
     if (!output.prepare()) {