Clear callbacks in camera stopPreview.

Clear those callbacks to remove the references
so the memory can be recycled.

bug:5358774
Change-Id: I3b6034a1891126b3437a6744013f38a64f5bb03b

Author: Chih-yu Huang

core/java/android/hardware/Camera.java

@@ -480,6 +480,12 @@ public interface PreviewCallback
     public final void stopPreview() {
         _stopPreview();
         mFaceDetectionRunning = false;
+
+        mShutterCallback = null;
+        mRawImageCallback = null;
+        mPostviewCallback = null;
+        mJpegCallback = null;
+        mAutoFocusCallback = null;
     }
 
     private native final void _stopPreview();