4/ Add basic drag and drop policy & handler for the shell

- Add basic policy for handling drags over home/fullscreen tasks
  /split tasks. For now, default to only allowing split to non-docked
  root task
- Refactor drag layout to have same lifecycle as the window instead of
  being per-drag session
- Resolve activity info for tasks as well as activity drag items
  (shortcuts to be resolved in a follow up CL)
- Fix crash due to sending drag events to all displays

Bug: 169894807
Test: atest DragAndDropPolicyTest

Change-Id: I7d777c59296e9ca9e1d169c8a021f9263af94d65