Allow appending static preferences to RadioButtonPickerFragment

This makes it so that static preferences can also be added after
the candidates so they can show up on the bottom of the screen.
This is done via a new xml tag for the preference screen that
is usually mostly empty that RaiodButtonPickerFragments use

Test: robotests
Bug: 111450127
Change-Id: I0fe2f480f0ff59b9bf9269e94b33ab4b008b47b8