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/libmediaplayerservice/MediaPlayerService.h b/media/libmediaplayerservice/MediaPlayerService.h
index 6b68b87..fa71d11 100644
--- a/media/libmediaplayerservice/MediaPlayerService.h
+++ b/media/libmediaplayerservice/MediaPlayerService.h
@@ -34,6 +34,7 @@
 
 namespace android {
 
+class AudioTrack;
 class IMediaRecorder;
 class IMediaMetadataRetriever;
 class IOMX;