Bug 1033549: [CostControl] Add a per-app data usage list

https://bugzilla.mozilla.org/show_bug.cgi?id=1033549
Dangerous. I'm not sure if this is the behavior we want, AFAIC, WiFi usage is not considered in the specs (that's why we are asking for clarifications on bugzilla). I would not take into account WiFi usage simply because it's free: I consider it is more informative for the user to know app ratios with the total **non-free** usage. What do you think?