Bug 828266:  [email] black flash / email flickers when transitioning from list to email and back

https://bugzilla.mozilla.org/show_bug.cgi?id=828266

This seems to do the trick for me on an unagi phone. I tried before this change and saw the black flash, after this change I do not see it.

I still see some dropping of animations for some transitions. I believe this is due to the size/complexity of the DOM for one of the cards. I will investigate a fix for that, but this change is the bare minimum for removing the black transition.