Add support for saving and using the Preference's key value

- make public and hide the PreferenceGroupAdapter class
- add setActivated(int position) to be able to set the activated item
- udpate getView(int position, View convertView, ViewGroup parent) to set/unset
the activated item

Change-Id: I8a653ad6541bb6b7a043ba7c3d95e5ecad924a0d