Fix bug that caused failure when all renderers disabled.

Prior to this change, there was a bug where playback would
fail with the following steps:

1. Start playback.
2. Pause playback.
3. Disable all renderers.
4. Enable at least one renderer.
5. Resume playback.

library/src/main/java/com/google/android/exoplayer/chunk/ChunkSampleSource.java

@@ -163,6 +163,7 @@ public class ChunkSampleSource implements SampleSource, SampleSourceReader, Load
     downstreamMediaFormat = null;
     downstreamPositionUs = positionUs;
     lastSeekPositionUs = positionUs;
+    pendingDiscontinuity = false;
     restartFrom(positionUs);
   }
 
@@ -170,7 +171,6 @@ public class ChunkSampleSource implements SampleSource, SampleSourceReader, Load
   public void disable(int track) {
     Assertions.checkState(state == STATE_ENABLED);
     Assertions.checkState(track == 0);
-    pendingDiscontinuity = false;
     state = STATE_PREPARED;
     try {
       chunkSource.disable(mediaChunks);
@@ -269,12 +269,14 @@ public class ChunkSampleSource implements SampleSource, SampleSourceReader, Load
   @Override
   public void seekToUs(long positionUs) {
     Assertions.checkState(state == STATE_ENABLED);
+
+    long currentPositionUs = isPendingReset() ? pendingResetPositionUs : downstreamPositionUs;
+    downstreamPositionUs = positionUs;
     lastSeekPositionUs = positionUs;
-    if ((isPendingReset() ? pendingResetPositionUs : downstreamPositionUs) == positionUs) {
+    if (currentPositionUs == positionUs) {
       return;
     }
 
-    downstreamPositionUs = positionUs;
     // If we're not pending a reset, see if we can seek within the sample queue.
     boolean seekInsideBuffer = !isPendingReset() && sampleQueue.skipToKeyframeBefore(positionUs);
     if (seekInsideBuffer) {

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

@@ -194,6 +194,7 @@ public class ExtractorSampleSource implements SampleSource, SampleSourceReader,
     if (enabledTrackCount == 1) {
       seekToUs(positionUs);
     }
+    pendingDiscontinuities[track] = false;
   }
 
   @Override
@@ -202,8 +203,8 @@ public class ExtractorSampleSource implements SampleSource, SampleSourceReader,
     Assertions.checkState(trackEnabledStates[track]);
     enabledTrackCount--;
     trackEnabledStates[track] = false;
-    pendingDiscontinuities[track] = false;
     if (enabledTrackCount == 0) {
+      downstreamPositionUs = Long.MIN_VALUE;
       if (loader.isLoading()) {
         loader.cancelLoading();
       } else {
@@ -274,13 +275,13 @@ public class ExtractorSampleSource implements SampleSource, SampleSourceReader,
       positionUs = 0;
     }
 
+    long currentPositionUs = isPendingReset() ? pendingResetPositionUs : downstreamPositionUs;
+    downstreamPositionUs = positionUs;
     lastSeekPositionUs = positionUs;
-    if ((isPendingReset() ? pendingResetPositionUs : downstreamPositionUs) == positionUs) {
+    if (currentPositionUs == positionUs) {
       return;
     }
 
-    downstreamPositionUs = positionUs;
-
     // 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.size(); i++) {

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

@@ -193,6 +193,7 @@ public class HlsSampleSource implements SampleSource, SampleSourceReader, Loader
     if (enabledTrackCount == 1) {
       seekToUs(positionUs);
     }
+    pendingDiscontinuities[track] = false;
   }
 
   @Override
@@ -201,8 +202,8 @@ public class HlsSampleSource implements SampleSource, SampleSourceReader, Loader
     Assertions.checkState(trackEnabledStates[track]);
     enabledTrackCount--;
     trackEnabledStates[track] = false;
-    pendingDiscontinuities[track] = false;
     if (enabledTrackCount == 0) {
+      downstreamPositionUs = Long.MIN_VALUE;
       if (loadControlRegistered) {
         loadControl.unregister(this);
         loadControlRegistered = false;
@@ -314,8 +315,11 @@ public class HlsSampleSource implements SampleSource, SampleSourceReader, Loader
   public void seekToUs(long positionUs) {
     Assertions.checkState(prepared);
     Assertions.checkState(enabledTrackCount > 0);
+
+    long currentPositionUs = isPendingReset() ? pendingResetPositionUs : downstreamPositionUs;
+    downstreamPositionUs = positionUs;
     lastSeekPositionUs = positionUs;
-    if ((isPendingReset() ? pendingResetPositionUs : downstreamPositionUs) == positionUs) {
+    if (currentPositionUs == positionUs) {
       return;
     }