Bug970187, CDMA-3way calls

https://bugzilla.mozilla.org/show_bug.cgi?id=970187
I'd prefer setting a class called `cdma-conference-call` on some element. And then do:
```css
.cdma-conference-call #group-show {
  display: none;
}
```
This is more consistent with how we are currently hiding that button: https://github.com/mozilla-b2g/gaia/blob/34cdda626d6bb7b0c74a82b4607162c2e967b80a/apps/callscreen/style/oncall.css#L816-L823