Bug 1043971 - Implement specific event for Loop calls "on hold"

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1043971 please.
You dont need this (it's in the CSS). However, when testing, we need to add to https://github.com/mozilla-b2g/firefoxos-loop-client/blob/master/call_screen/style/oncall.css#L699 one more condition, 'body[data-call-status='hold'] .local-bubble-shadow' in order to work properly.