Refactoring the focus state to be independent of view focus.

- This allows us to ensure that a task is always focused in the stack,
  which simplified focus logic, and allows us to unify the accessibility
  focus and normal view focus handling.
- Refactoring some focus actions to events
- Adding flags to experiment with fast-toggle.

Change-Id: I5623f2a5584994033f1ebf1760b55e8bd019944b