Work on DialogFragment and docs.

- DialogFragment now has an option to not show a dialog, so you can
  use your UI somewhere else.
- Deprecated show() versions that were tied to activities.
- Added documentation to DialogFragment class.
- Added documentation to onSaveInstanceState() to explain how the
  time it is called is different than Activity's version.
- Fixed some java doc warnings.

Change-Id: If026744c368e2443030d2d9e0a9c808d820857df