Give MSG constants different values.

This should be a no-op change, but it seems sensible to
have them not overlap.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123208140

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

@@ -241,7 +241,7 @@ public interface C {
    * {@link ExoPlayer#sendMessage} or {@link ExoPlayer#blockingSendMessage}. The message object
    * should be a {@link Float} with 0 being silence and 1 being unity gain.
    */
-  int MSG_SET_VOLUME = 1;
+  int MSG_SET_VOLUME = 2;
 
   /**
    * The type of a message that can be passed to an audio {@link TrackRenderer} via
@@ -250,6 +250,6 @@ public interface C {
    * underlying {@link android.media.AudioTrack}. The message object should not be modified by the
    * caller after it has been passed
    */
-  int MSG_SET_PLAYBACK_PARAMS = 2;
+  int MSG_SET_PLAYBACK_PARAMS = 3;
 
 }