Bug 1078243 - [Costcontrol] When the Starting day of the reset period is...

... 31th, the app does not calculated correctly the end of the reset period.
Per [line 65](https://github.com/mozilla-b2g/gaia/pull/25267/files#diff-253f67f98654de3942030acdc52e650fR65), `lowerDate` is already `Toolkit.toMidnight(new Date())`.