Bug 962451 - [email] Start from activity, tap back from compose/message reader, empty message list

The comment in the code change has most of the details. The main issue: the message_list card is created after header_cursor already has a folder and a messages slice, so the message_list card will not get any event notifications of the existing messages slice data.

I had thought about triggering this sort of manual update in [mail_app's Cards.pushDefaultCard implementation](https://github.com/mozilla-b2g/gaia/blob/d425a84fd2b92389d4daaff8f57d8f5f54ef35bc/apps/email/js/mail_app.js#L107), but feel it is better if message_list is self-contained, and can generate its view without needing an exterior kick.

In my fancy future ideal, header_cursor would know to replay events for any new subscribers so that this sort of manual bootstrapping is not required, but that requires more thought, and may be more appropriate to consider as part of [bug 937959](https://bugzilla.mozilla.org/show_bug.cgi?id=937959), or may be invalidated by that work.