[NS08] Expose public NetworkScore API

This exposes the two bits settable by the network agents in
Android S on NetworkScore. This is meant to be extensible in
future releases, or possibly for OEM upstreams.

Test: builds
CTS-Coverage-Bug: 184037351
Bug: 167544279
Change-Id: Id7ee1bd3e2679f0bd5200d5c299f18a33b87847c