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

https://bugzilla.mozilla.org/show_bug.cgi?id=1033549
In combination with the inner function, the name is a little bit misleading as the inner method does not limit to get **one** sample but the samples for an specified application so please, could yo change these names to (for instance): 

`requestSamples()` to `requestPerAppUsage()` and
`requestSample()` to `requestSamples()`