[MS16.1] Prepare APIs for replacing getNetwork[Total|Uid]Bytes

These internal methods are used in MultipathPolicyTracker and
NetworkPolicyManagementService. getNetworkTotalBytes is used
to query total bytes regarding to the specified NetworkPolicy,
to see how much data quota is used. getNetworkUidBytes is used
to find the app to blame for rapid data usage.

These methods needs to be replaced by public APIs in follow-up
patches.

Test: TH
Bug: 204830222
Change-Id: Iec9cc7a07397ec2bbc6c3d2559fc3680b149cb8a