Discard preparation chunk if track selection does not include it

This avoids breaking the player if the first variant is not
supported by the device.

Issue:#2353

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148025791

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

@@ -182,6 +182,7 @@ import java.util.LinkedList;
       }
     }
     // Enable new tracks.
+    TrackSelection primaryTrackSelection = null;
     boolean selectedNewTracks = false;
     for (int i = 0; i < selections.length; i++) {
       if (streams[i] == null && selections[i] != null) {
@@ -189,6 +190,7 @@ import java.util.LinkedList;
         int group = trackGroups.indexOf(selection.getTrackGroup());
         setTrackGroupEnabledState(group, true);
         if (group == primaryTrackGroupIndex) {
+          primaryTrackSelection = selection;
           chunkSource.selectTracks(selection);
         }
         streams[i] = new HlsSampleStream(this, group);
@@ -205,6 +207,14 @@ import java.util.LinkedList;
           sampleQueues.valueAt(i).disable();
         }
       }
+      if (primaryTrackSelection != null && !mediaChunks.isEmpty()) {
+        primaryTrackSelection.updateSelectedTrack(0);
+        int chunkIndex = chunkSource.getTrackGroup().indexOf(mediaChunks.getLast().trackFormat);
+        if (primaryTrackSelection.getSelectedIndexInTrackGroup() != chunkIndex) {
+          // The loaded preparation chunk does match the selection. We discard it.
+          seekTo(lastSeekPositionUs);
+        }
+      }
     }
     // Cancel requests if necessary.
     if (enabledTrackCount == 0) {