Define API for metering network stats buckets.

This is a continuation of b/26545374, since now we can also set the
metered bit with NetworkScorer. The tracking of metered state changes
will be implemented in a seperate CL.

Bug: 31015360
Bug: 26545374
Test: N/A
Change-Id: I0eccd10c0316357abb03af4d9cf0c4191a66abfb