NetworkStatsService: Fix getDetailedUidStats to take VPNs into account.

This API is similar to one provided by NetworkStatsFactory with the
difference that NSS also migrates traffic from VPN UID to other apps.

Since traffic can only be migrated over NetworkStats delta, NSS
therefore maintains NetworkStats snapshot across all UIDs/ifaces/tags.

This snapshot gets updated whenever NSS records a new snapshot
(based on various hooks such as VPN updating its underlying networks,
network getting lost, etc.), or getDetailedUidStats API is invoked by
one of its callers.

Bug: 113122541
Bug: 120145746
Test: atest FrameworksNetTests
Test: manually verified that battery stats are migrating traffic off of
TUN (after patching above CL where we point BatteryStats to use this
API).
Change-Id: Ib0f0c2d4d41ee1d7a027ea9da457baaf198d649e