Bug 1070808 - Take SIM card 2 into consideration when checking voicemail number



https://bugzilla.mozilla.org/show_bug.cgi?id=1070808
Is there a reason to allow mutating different settings keys? It seems like the only one that gets used here is ```ril.iccInfo.mbdn```, so we should just set that here. See https://github.com/mozilla-b2g/gaia/pull/24429/files#r18192347