Add mechanism for a task to be hidden as a part of starting a drag

- This change adds a new DRAG_FLAG_HIDE_CALLING_TASK_ON_DRAG_START
  (restricted to the assistant uid) which will allow the calling task's
  window to hide it's surface for the duration of the drag.  As a part
  of setting this flag, the return/cancel animations are also adjusted
  to fade out in-place.

  In order to handle most of this logic in the shell instead of core,
  the drag & drop flow will notify (only) the shell of the drag source
  task id (if it has requested the flag to hide), and the shell will
  in turn hide the task surface and upon successful drop to invoke
  splitscreen, the shell will reorder the task to the back. If the drag
  is not handled or canceled, the task surface is made visible again.

Bug: 350016003
Flag: EXEMPT bugfix
Test: Drag floating window to split, ensure it hides and restores if the
      drag is canceled
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:b51bba4b02d02b1f3cbd244d956e1231dc5a4b65)
Merged-In: Id96ef40a6f74892eac22b9ef3c04107b1b07cfb2
Change-Id: Id96ef40a6f74892eac22b9ef3c04107b1b07cfb2