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.
Bug: b/72107146
Merged-In: I31c5b9b4a7c6e72910152415894a137f000a5858
Merged-In: I2527d95000c7500c824ede70f87ecb38e21ed323
(cherry picked from aosp 088ff682)

Change-Id: Ie80f1bb21124241f3414f9be77aceac9a44ec6d1