Add all-channel AudioTrack::setVolume() API

Add combined channel APIs setVolume to AudioTrack,
and remove obsolete getVolume.

Change-Id: I0c87bfdbff4f4292259fa33e65f67badbafd270b
diff --git a/include/media/AudioTrack.h b/include/media/AudioTrack.h
index 3d45503..6d19092 100644
--- a/include/media/AudioTrack.h
+++ b/include/media/AudioTrack.h
@@ -285,9 +285,14 @@
 
     /* Set volume for this track, mostly used for games' sound effects
      * left and right volumes. Levels must be >= 0.0 and <= 1.0.
+     * This is the older API.  New applications should use setVolume(float) when possible.
      */
             status_t    setVolume(float left, float right);
-            void        getVolume(float* left, float* right) const;
+
+    /* Set volume for all channels.  This is the preferred API for new applications,
+     * especially for multi-channel content.
+     */
+            status_t    setVolume(float volume);
 
     /* Set the send level for this track. An auxiliary effect should be attached
      * to the track with attachEffect(). Level must be >= 0.0 and <= 1.0.