Fix a couple of drag and drop issues from transient taskbar

- Don't stash until drag ends, but immediately stash at that point
  (regardless of success or failure, though failure will animate to the
  original icon before stashing)
- Send transient taskbar's bounds to WM Shell via intent extra such that
  they ignore drag events in that region

Test: manual in persistent and transient taskbar
Bug: 269814838
Fixes: 268526633
Fixes: 259645384
Change-Id: I5ded3998046f259ed6e79cb4ed765ad7b0c72e45