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.
Mmm, if you mean moving ```MessageManager.markThreadRead``` call into ```ThreadListUI.mark``` then, yeah, maybe in the next patch that will touch this code, but if you mean vice versa then I wouldn't do this as ```ThreadListUI.mark``` is UI only thing that ```MessageManager.markThreadRead``` shouldn't know about :)