Bug 1033403 - [Messages] We don't stop rendering if we tap "back" too so...

...on r=azasypkin

This moves the moment "initializeRendering" is called: in beforeEnter instead of
afterEnter.
I did that first, then I changed stuff to call onMessageDone inside the if-block, so I thought that it was better to define functions first (the "real" functions would be anyway, but the "bind" calls could rightly be avoided). When I reverted the change, I left the if-block here for this reason. But it's arguable.