A few bug fixes.

1. Suppress onContinueLoadingRequested until a source is
   prepared (since such calls are not supposed to be made
   until after preparation has completed).
2. Don't use the format evaluator in HlsChunkSource prior
   to prepration completing.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=126794616

library/src/main/java/com/google/android/exoplayer/MultiSampleSource.java

@@ -192,6 +192,10 @@ public final class MultiSampleSource implements SampleSource, SampleSource.Callb
 
   @Override
   public void onContinueLoadingRequested(SampleSource ignored) {
+    if (trackGroups == null) {
+      // Still preparing.
+      return;
+    }
     callback.onContinueLoadingRequested(this);
   }
 

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

@@ -75,7 +75,6 @@ public class HlsChunkSource {
   private final Variant[] variants;
   private final HlsMediaPlaylist[] variantPlaylists;
   private final long[] variantLastPlaylistLoadTimesMs;
-  private final int initialEnabledVariantIndex;
 
   private boolean seenFirstExternalTrackSelection;
   private byte[] scratchSpace;
@@ -119,7 +118,6 @@ public class HlsChunkSource {
       initialTrackSelection[i] = i;
     }
     selectTracksInternal(initialTrackSelection, false);
-    initialEnabledVariantIndex = getEnabledVariantIndex(variants[0].format);
   }
 
   /**
@@ -440,6 +438,10 @@ public class HlsChunkSource {
     enabledVariantBlacklistTimes = new long[enabledVariants.length];
     enabledVariantBlacklistFlags = new boolean[enabledVariants.length];
 
+    if (!isExternal) {
+      return;
+    }
+
     if (enabledVariants.length > 1) {
       Format[] formats = new Format[enabledVariants.length];
       for (int i = 0; i < formats.length; i++) {
@@ -458,11 +460,26 @@ public class HlsChunkSource {
 
   private void updateFormatEvaluation(HlsMediaChunk previous, long playbackPositionUs) {
     clearStaleBlacklistedVariants();
-    if (!seenFirstExternalTrackSelection
+    if (!seenFirstExternalTrackSelection) {
-        && !enabledVariantBlacklistFlags[initialEnabledVariantIndex]) {
+      if (!enabledVariantBlacklistFlags[getEnabledVariantIndex(variants[0].format)]) {
         // Use the first variant prior to external track selection, unless it's been blacklisted.
         evaluation.format = variants[0].format;
-    } else if (enabledVariants.length > 1) {
+        return;
+      }
+      // Try from lowest bitrate to highest.
+      for (int i = enabledVariants.length - 1; i >= 0; i--) {
+        if (!enabledVariantBlacklistFlags[i]) {
+          evaluation.format = enabledVariants[i].format;
+          return;
+        }
+      }
+      // Should never happen.
+      throw new IllegalStateException();
+    }
+    if (enabledVariants.length == 1) {
+      evaluation.format = enabledVariants[0].format;
+      return;
+    }
     long bufferedDurationUs;
     if (previous != null) {
       // Use start time of the previous chunk rather than its end time because switching format
@@ -473,9 +490,6 @@ public class HlsChunkSource {
     }
     adaptiveFormatEvaluator.evaluateFormat(bufferedDurationUs, enabledVariantBlacklistFlags,
         evaluation);
-    } else {
-      evaluation.format = enabledVariants[0].format;
-    }
   }
 
   private boolean shouldRerequestLiveMediaPlaylist(int variantIndex) {

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

@@ -284,6 +284,10 @@ public final class HlsSampleSource implements SampleSource,
 
   @Override
   public void onContinueLoadingRequested(HlsTrackStreamWrapper trackStreamWrapper) {
+    if (trackGroups == null) {
+      // Still preparing.
+      return;
+    }
     callback.onContinueLoadingRequested(this);
   }