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

https://bugzilla.mozilla.org/show_bug.cgi?id=1033549
We are breaking behavior. For some reason, some time before I was not able to store all the historical data (it breaks often) so I only store totals. If your remove this, please move the former `setOption()` block on line 498 to continue storing a proper `lastDataUsage` with totals. If not, totals will be 0 and the correct `lastDataUsage` will be never persisted.