Add unit tests related to data accounting for VPNs with one underlying

network.

This is to establish a baseline for the existing behavior, and to ensure
that following changes are not causing a regression in existing
behavior.

This CL is also adding missing cleanup for NetworkStatsCollectionTest
which was forcing all network types in NetworkTemplate that was causing
NetworkStatsService related tests to fail.

Bug: 113122541
Bug: 120145746
Test: atest FrameworksNetTests
Change-Id: I285f186cfb16bc9fa704c797996b1e4f8a73dee4