Define new APIs for metering and roaming network stats buckets.

These are analagous to the state buckets for tracking whether usage is
incurred while the app is in the foreground or background. We will
additionally track whether data usage is incurred over a metered or
unmetered network, and whether it is incurred over a roaming or native
network.

The APIs are not implemented in this CL; the existing buckets are
still returned with METERING_ALL and ROAMING_ALL to indicate that this
is not yet being tracked.

Bug: 25813438
Bug: 25813958
Change-Id: I76dd3dd063ed28ef5579ca3a978570532e7836bc