Bug 1015297 - [Settings] Update to use gaia-header

Because you fixed the dom structure here, you have to fix https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/js/call.js#L91-L92

In single sim, we can just go back to `#root` and it's fine, but in DSDS, we have to override `data-href` in `<gaia-header>` to make sure we can go back to `#call-iccs` panel.