Proposed fix for issue 6256 (stale TrackSelection reference)

Move the update of the HlsChunkSource's TrackSelection reference to be seperate from the loop that recreates HlsSampleStreams.  This way the reference is update even if the input HlsSampleStream reference was not nulled out (retain the stream when a new TrackSelection is created that is essentially a copy of the previous one.

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

@@ -270,14 +270,32 @@ public boolean selectTracks(TrackSelection[] selections, boolean[] mayRetainStre
       SampleStream[] streams, boolean[] streamResetFlags, long positionUs, boolean forceReset) {
     Assertions.checkState(prepared);
     int oldEnabledTrackGroupCount = enabledTrackGroupCount;
-    // Deselect old tracks.
+
+    // Get the old (i.e. current before the loop below executes) primary track selection. The new
+    // primary selection will equal the old one unless it's changed in the loop.
+    TrackSelection oldPrimaryTrackSelection = chunkSource.getTrackSelection();
+    TrackSelection primaryTrackSelection = oldPrimaryTrackSelection;
+
     for (int i = 0; i < selections.length; i++) {
-      if (streams[i] != null && (selections[i] == null || !mayRetainStreamFlags[i])) {
+      TrackSelection selection = selections[i];
+
+      // Deselect old tracks, discarding the sample stream unless mayRetainStream is set
+      if (streams[i] != null && (selection == null || !mayRetainStreamFlags[i])) {
         enabledTrackGroupCount--;
         ((HlsSampleStream) streams[i]).unbindSampleQueue();
         streams[i] = null;
       }
+
+      // update the possibly changed reference to the track selection for the primary track group
+      if (selection != null) {
+        int trackGroupIndex = trackGroups.indexOf(selection.getTrackGroup());
+        if (trackGroupIndex == primaryTrackGroupIndex) {
+          primaryTrackSelection = selection;
+          chunkSource.selectTracks(selection);
+        }
+      }
     }
+
     // We'll always need to seek if we're being forced to reset, or if this is a first selection to
     // a position other than the one we started preparing with, or if we're making a selection
     // having previously disabled all tracks.
@@ -286,20 +304,12 @@ public boolean selectTracks(TrackSelection[] selections, boolean[] mayRetainStre
             || (seenFirstTrackSelection
                 ? oldEnabledTrackGroupCount == 0
                 : positionUs != lastSeekPositionUs);
-    // Get the old (i.e. current before the loop below executes) primary track selection. The new
-    // primary selection will equal the old one unless it's changed in the loop.
-    TrackSelection oldPrimaryTrackSelection = chunkSource.getTrackSelection();
-    TrackSelection primaryTrackSelection = oldPrimaryTrackSelection;
+
     // Select new tracks.
     for (int i = 0; i < selections.length; i++) {
       if (streams[i] == null && selections[i] != null) {
         enabledTrackGroupCount++;
-        TrackSelection selection = selections[i];
-        int trackGroupIndex = trackGroups.indexOf(selection.getTrackGroup());
-        if (trackGroupIndex == primaryTrackGroupIndex) {
-          primaryTrackSelection = selection;
-          chunkSource.selectTracks(selection);
-        }
+        int trackGroupIndex = trackGroups.indexOf(selections[i].getTrackGroup());
         streams[i] = new HlsSampleStream(this, trackGroupIndex);
         streamResetFlags[i] = true;
         if (trackGroupToSampleQueueIndex != null) {