Bug 944235 - Modify the CostControl app of gaiatest

The Bug 937244 added three buttons to delete data usages of Wifi/Mobile.

ref: https://github.com/mozilla-b2g/gaia/commit/f20aee1acb1bc3612bd42330c25f03f9c5a5d4bd#diff-436d80a6049b12e0a07272f67afd2ab4