Re-use existing group and duplicates fragments if possible

- If group fragment is added and visible and we are loading another
  group, we simply update the existing group fragment.
- Always see if a fragment is in fargment manager before creating and
  adding another one.
- Scroll to top when switching to all contacts view.
- Address a TODO to pop group fragment rather than finishing activity.

Bug: 30944495

Test: manual - navigate b/w fragments (w/ rotation), add and remove
               groups; and observe fragment manager's behavior.

Change-Id: Idff71e62ed6ff6e2a12884b2f232336fe3edbf38