Fix memory leak in Launcher

Leak happened when a stack view was on the
workspace and then workspace forced a rebind of
its items (for example, when an app was installed)

Bug # 5629171
diff --git a/src/com/android/launcher2/Launcher.java b/src/com/android/launcher2/Launcher.java
index f9c7bf0..864fca5 100644
--- a/src/com/android/launcher2/Launcher.java
+++ b/src/com/android/launcher2/Launcher.java
@@ -2920,6 +2920,7 @@
             final CellLayout layoutParent = (CellLayout) workspace.getChildAt(i);
             layoutParent.removeAllViewsInLayout();
         }
+        mWidgetsToAdvance.clear();
         if (mHotseat != null) {
             mHotseat.resetLayout();
         }