DO NOT MERGE: 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)

(cherry picked from commit dadf6caf)

Change-Id: I01ebb270c5bf7f2383e8923cce72545ca1bee118