Improvements to fragment API:

- Remove names associated with fragments (we'll use ids).  They may
  come back for other uses.
- Add method to replace a fragment, to help in doing back stacks.
- Add names to back stack states, to allow for jumping back to
  known states.
- Add ability to set animations associated with fragments being
  added or removed.

Change-Id: Ic2799f06947b1511fb1df5965ca32568b860b3c3