Fix: Bug 1069599

Changed all gaia-subheaders in costcontrol to aria-level 2 with role="heading".

Bug can be found at https://bugzilla.mozilla.org/show_bug.cgi?id=1069599

@lodr I was asked to have you review this PR.
AFAIK, `role` attribute does not exist, it is `aria-role` but adding it could affect the style.