SelectPaymentMethodSlave: Fix some problems introduced by my last patch

* When the selected method is not sensitive, select the default (or fallback to
  money) instead of letting it selected. This fixes a problem when the
  DEFAULT_PAYMENT_METHOD is credit/store_credit and the radio is visible but
  not sensitive.
* On renegotiationwizard use set_client instead of duplicating the code. This
  makes it fix the problem above by itself.
* Remove some not used apis. One was being used on tests so make them use the
  code without the api. Since we cannot have a non visible/sensitive widget, we
  would have to prevent a lot of problems on that api (raise an error if those
  conditions were met, and things like that).
* Write some tests that made the code 100% covered just by the test module

Change-Id: Id93cee33e252796952ca72d51e4c882e566daf31