Minor tweaks to AudioTrack playback params logic.

- Use allowDefaults to fix crash if params are passed without
  the speed being explicitly set.
- Allow null to be passed to clear previously set params.
- Clarify in doc that the passed params shouldn't be modified
  after they're passed.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=109591580

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

@@ -80,8 +80,9 @@ public class MediaCodecAudioTrackRenderer extends MediaCodecTrackRenderer implem
   /**
    * The type of a message that can be passed to an instance of this class via
    * {@link ExoPlayer#sendMessage} or {@link ExoPlayer#blockingSendMessage}. The message object
-   * should be a {@link android.media.PlaybackParams}. This will be used to configure the
-   * underlying {@link android.media.AudioTrack}.
+   * should be a {@link android.media.PlaybackParams}, which will be used to configure the
+   * underlying {@link android.media.AudioTrack}. The message object should not be modified by the
+   * caller after it has been passed
    */
   public static final int MSG_SET_PLAYBACK_PARAMS = 2;
 

library/src/main/java/com/google/android/exoplayer/audio/AudioTrack.java

@@ -1193,28 +1193,36 @@ public final class AudioTrack {
   private static class AudioTrackUtilV23 extends AudioTrackUtilV19 {
 
     private PlaybackParams playbackParams;
+    private float playbackSpeed;
+
+    public AudioTrackUtilV23() {
+      playbackSpeed = 1.0f;
+    }
 
     @Override
     public void reconfigure(android.media.AudioTrack audioTrack,
         boolean needsPassthroughWorkaround) {
       super.reconfigure(audioTrack, needsPassthroughWorkaround);
-      setPlaybackParameters(playbackParams);
+      maybeApplyPlaybackParams();
     }
 
     @Override
     public void setPlaybackParameters(PlaybackParams playbackParams) {
+      playbackParams = (playbackParams != null ? playbackParams : new PlaybackParams())
+          .allowDefaults();
       this.playbackParams = playbackParams;
-      if (audioTrack != null && playbackParams != null) {
-        audioTrack.setPlaybackParams(playbackParams);
-      }
+      this.playbackSpeed = playbackParams.getSpeed();
+      maybeApplyPlaybackParams();
     }
 
     @Override
     public float getPlaybackSpeed() {
-      if (playbackParams != null) {
-        return playbackParams.getSpeed();
-      } else {
-        return 1.0f;
+      return playbackSpeed;
+    }
+
+    private void maybeApplyPlaybackParams() {
+      if (audioTrack != null && playbackParams != null) {
+        audioTrack.setPlaybackParams(playbackParams);
       }
     }