Fixing issue with wrong transition when tasks are offscreen.

- Fixing issue with wrong animation specs being created for tasks that
  are offscreen
- Removing unused code and simplifying the logic for task launching, also
  pulling this out into another file so that it is easier to read and
  understand
- Removing old incorrect call to moveTaskToFront() instead of
  startActivityFromRecents()

Bug: 25590404

Change-Id: I25d9530d089a7984fb8c94954a34dd124420755a