MediaPlayer: add a disconnect from ANativeWindow

This change makes MediaPlayer::disconnect disconnect from the
ANativeWindow once the player has been disconnected.

Change-Id: I6744559030d1edc520cf77402d1f5ce841742340
diff --git a/media/libmedia/mediaplayer.cpp b/media/libmedia/mediaplayer.cpp
index 178039c..ed6e3c7 100644
--- a/media/libmedia/mediaplayer.cpp
+++ b/media/libmedia/mediaplayer.cpp
@@ -84,6 +84,8 @@
     if (p != 0) {
         p->disconnect();
     }
+
+    disconnectNativeWindow();
 }
 
 // always call with lock held