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

(cherry picked from commit 9ac251d1)

Change-Id: I6a2fcc8577d62daa8823dff9e0128a5cf39ede7b