Bring HTMLSelectAndOtherField in line with other HTMLFormFields by
ensuring that the default value is of the correct type and passes
validation checks.
Also make sure HTMLSelectAndOtherField::validate checks for the
final value, if the field has required set to true.
Bug: T222170
Bug: T225860
Change-Id: I949ee3df2b1f597982cf522b149c54b8e79d59bc