Chosen list of acceptable algorithms neither saves nor validates

When editing the field settings, the chosen algorithms don't persist across saves, and the keys entered aren't validated against this list. So there are really two problems here, not saving config, and validations not applying.

Steps to reproduce:

1. Add an SSH Key field to an entity.
2. Choose some algorithms in the field settings.
3. Save field settings.
4. Edit the field settings again.
5. Notice that all of the boxes are unchecked, even though you previously checked some.
6. Have a user save a key for an unchecked algorithm.
7. Notice that it's accepted, without a form validation failure.

Remaining tasks:

1. Get the field config saving properly, and
2. actually validate the form field against the chosen algorithms.