Dev preview changes for incompatible apps.

- Disable drop targets when picking up a task that can’t be docked, and
  show toast notifying user (for now)
- When a task is docked, update the task headers for non-dockable tasks
  to indicate that they don’t support multi window.  Update these 
  headers every time the multi window state changes.

Bug: 26774190
Change-Id: I020dcbac9b1230dba166eecaa12366986e684f9e