1489393 - fix disappearing contacts

Fixes pressing the minus key in the contacts list by making sure that it only expands when it has children and only collapses when the selected message is a root (and it's expanded).  I initially tried to just put the expand/collapse thread functionality in the message list key handler in app.js, but it turned out to be a lot more changes.  Also doing it this way ensures that if if the contact list ever needs a tree structure (though I don't know why it would) it will be supported.