Bug 977056 - [Dialer] Visual discrepancies: Call ended view under conference call participants list

The main reason is to avoid the parent node of the call node to be removed to be null. This can happen when the list of participants of a conference call is shown, a call participating in it hangs up (its disconnect event is notified and the call node removal is requested after some seconds to let user see the call ending transition, see https://github.com/mozilla-b2g/gaia/blob/master/apps/callscreen/js/handled_call.js#L326) but before those seconds take place the user decides to click on the button to hide the participant list. The closing of the participant list also asks for the removal of the already ended calls from the main call screen page so the user is presented with the proper new call screen layout without seeing the ending call transitions. I could remove the removal of the already ended calls from the call screen when the user hides the participant list and that checking should not be necessary. In fact, you can see both effects just deleting https://github.com/gtorodelvalle/gaia/blob/callscreen-bug-977056-participant-leaving-conference/apps/callscreen/js/conference_group_ui.js#L53 in my patch.