Remove activity and window stacks when last task is removed.

Also,
* Only restore recent task to a full screen stack instead of any
  randomly sized stack.
* Fixed issue where we were restore task from recents for some
  operations when we didn't need to.
* Null out TaskRecord.stack when the task is removed from the
  stack.

Bug: 19083170
Change-Id: I4e006f101f9d0f2aebde130ad77afc4d258c6612