Fix incorrect includes of AudioTrack.h

Remove unnecessary includes of AudioTrack.h.
Use forward declaration of class names in preference to #include when possible.

Change-Id: I12982811fa75c2c7695d8bbfa595a7aaec047dc0
diff --git a/media/libmedia/mediaplayer.cpp b/media/libmedia/mediaplayer.cpp
index acf97a6..f1c47dd 100644
--- a/media/libmedia/mediaplayer.cpp
+++ b/media/libmedia/mediaplayer.cpp
@@ -30,7 +30,7 @@
 #include <gui/SurfaceTextureClient.h>
 
 #include <media/mediaplayer.h>
-#include <media/AudioTrack.h>
+#include <media/AudioSystem.h>
 
 #include <surfaceflinger/Surface.h>