Add one more test for VPN usage stats.

Covers the case where the majority of traffic through the VPN is
caused by the VPN app itself, and ensures that that traffic is
correctly attributed to the VPN app as opposed to spread between
the other apps that use the VPN.

Bug: 120145746
Test: atest NetworkStatsServiceTest
Change-Id: Iffd3f95fc2e11d311691a797b010edb38d2ef3c6