mediaplayer: support dynamic playback rate

Bug: 19196501

Change-Id: I856b1507d5fa2cedfb645706d2435683a7d3e050
diff --git a/include/media/IMediaPlayer.h b/include/media/IMediaPlayer.h
index db62cd5..4153c25 100644
--- a/include/media/IMediaPlayer.h
+++ b/include/media/IMediaPlayer.h
@@ -56,6 +56,7 @@
     virtual status_t        stop() = 0;
     virtual status_t        pause() = 0;
     virtual status_t        isPlaying(bool* state) = 0;
+    virtual status_t        setPlaybackRate(float rate) = 0;
     virtual status_t        seekTo(int msec) = 0;
     virtual status_t        getCurrentPosition(int* msec) = 0;
     virtual status_t        getDuration(int* msec) = 0;