Clear ref to animation after drag & drop animation completes

- LauncherAppTransitionManagerImpl -> DragLayer -> mDropAnim -> mDragView
  (via the update listener) -> mBitmap

Bug: 76162466
Test: Take hprof after picking up an app or widget, ensure there are no
      references to the image created for drag and drop

Change-Id: Iffc92eed13775921567f9ac25fea8e356d6fe963

src/com/android/launcher3/dragndrop/DragLayer.java

@@ -1,3 +1,4 @@
+
 /*
  * Copyright (C) 2008 The Android Open Source Project
  *
@@ -479,6 +480,7 @@ public class DragLayer extends BaseDragLayer<Launcher> {
                 case ANIMATION_END_REMAIN_VISIBLE:
                     break;
                 }
+                mDropAnim = null;
             }
         });
         mDropAnim.start();
@@ -488,6 +490,7 @@ public class DragLayer extends BaseDragLayer<Launcher> {
         if (mDropAnim != null) {
             mDropAnim.cancel();
         }
+        mDropAnim = null;
         if (mDropView != null) {
             mDragController.onDeferredEndDrag(mDropView);
         }