Fix showing wrong image on document picker item

Set mDetachedRequestDescriptor as null for document picker to prevent
calling setImageResourceId() in onAttachedToWindow().

Test: Manual

Change-Id: Ied5ca200cac350ef4b2420269763dfc240fc97c4
Signed-off-by: Taesu Lee <taesu82.lee@samsung.com>

src/com/android/messaging/ui/AsyncImageView.java

@@ -125,6 +125,8 @@ public class AsyncImageView extends ImageView implements MediaResourceLoadListen
                 return;
             }
             unbindView();
+        } else {
+            mDetachedRequestDescriptor = null;
         }
         setImage(null);
         resetTransientViewStates();