Fixing crash when trying to reset focused task after removing task.

- Moving the focused state back into the stack, since the task view
  is transient and can be rebound.  Also ensuring that we update the
  task view focus state as we return and pick up views from the view
  pool
- Fixing issue where going back from recents would not go back home
- Properly calculating the task visibility (regression)

Bug: 25975225

Change-Id: Ica861f0d8996e4e254e875199a2d704a65bf8e58