1/ Add basic infra for global drag handling in the Shell

- Add private window flag INTERCEPT_GLOBAL_DRAG_AND_DROP (protected by
  MANAGE_ACTIVITY_STACKS permission) to be considered as a part of the
  normal flow for global drag and drops even if the window is not
  visible.  In addition, the window with the flag receives the clip
  data on DRAG_STARTED and the drag surface on DROP.  If the window
  consumes the drop, then it will relinquish cleanup of the drag surface
  from the system.
- Add MIMETYPE_APPLICATION_ACTIVITY for an app to report that they are
  starting a global drag of an activity. The associated data must
  include an intent with the pending intent and user to launch the
  activity for.
- Add a test drag handler on the shell end to receive the drag, setup a
  drag layout, and just launch the intent (for now).

Bug: 169894807
Test: atest DragDropControllerTest
Test: atest DragDropTest

Change-Id: I7f5cdca3cf515b693a8f1e507e90e22a670b5fa6
Merged-In: I7f5cdca3cf515b693a8f1e507e90e22a670b5fa6
Signed-off-by: Winson Chung <winsonc@google.com>