Add method to NetworkStatsService for UID stats.

Useful for clients such as BatteryStats which currently rely
on NetworkStatsFactory. Data at that stage is incomplete as
it does not account for tethering, VT data and corresponding
464xlat corrections.

Test: runtest frameworks-net, CTS tests pass.
Change-Id: I763b77f601c827fd2963204694fb5b45425cc791