Added expiration timer to data plan that fires when the data plan expires, and assigns it the bytes used since the last byte counter update.

BUG=chromium-os:19343
TEST=run basic manual tests on chrome book

Change-Id: I8f014f4c228081f32048dcc820ffb8c988be0e0c
Reviewed-on: http://gerrit.chromium.org/gerrit/6229
Reviewed-by: Vince Laviano <vlaviano@chromium.org>
Tested-by: Michael Mekonnen <mikemeko@chromium.org>