Clear WidgetCell#mItem before recylcing a WidgetCell

This fixes a bug which reuses a previous widget item spans to
compute the preview size.

Fix: 193133048
Test: manual
Change-Id: I5459882e258dc010898854dbbe52a6fa8d84796c
(cherry picked from commit 3d27207ff6ff951add0a21b699761126d1af11ec)
diff --git a/src/com/android/launcher3/widget/WidgetCell.java b/src/com/android/launcher3/widget/WidgetCell.java
index c3edcce..6605509 100644
--- a/src/com/android/launcher3/widget/WidgetCell.java
+++ b/src/com/android/launcher3/widget/WidgetCell.java
@@ -177,6 +177,7 @@
             mWidgetImageContainer.removeView(mAppWidgetHostViewPreview);
         }
         mAppWidgetHostViewPreview = null;
+        mItem = null;
     }
 
     public void setSourceContainer(int sourceContainer) {