Pushing the startActivity before the calls to remove task.

- It appears that there is contention between startActivityAsUser() and
  removeTask() (called on two separate threads) which can cause jank when
  a user removes all the tasks from their recents list.  This CL ensures
  that startActivityAsUser() is always run first so it is not blocked
  by the other call (which should be able to run in the background
  uninterrupted).

Bug: 22760556
Change-Id: I7564a2f0e43414686419d3657379bbd0ca6b4152