Defer calling the stack task removed callbacks.

- When updating a TaskStack, defer calling any callbacks until the stack
  is updated.  Otherwise, the handler may incorrectly check the size of
  the stack to be empty, which triggers Recents to go home.
- This CL also removes an erroneous double call to onStackTaskRemoved()
  when updating, one explicit call and the call that is made internally
  in removeTask().

Bug: 28941269
  

Change-Id: I37b93bd959a625096925a16a426269a0c79de4e7