cashew: rename UsageRequestStatus metric

This CL renames the Cros.Network.Cashew.UsageRequestStatus metric to
Network.Cellular.UsageRequestStatus to better conform with existing
conventions.

BUG=chromium-os:9887
TEST=unit tests, manual testing on device

Change-Id: I681f7f7e17493805723d8a51130ccdce709698c5
Reviewed-on: http://gerrit.chromium.org/gerrit/5363
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Vince Laviano <vlaviano@chromium.org>