Add VCN underlying networks

This change declares underlying networks for the VCN for the purposes of
app data accounting

Bug: 190620024
Test: atest FrameworksVcnTests
Original-Change: https://android-review.googlesource.com/1750241
Merged-In: Ida2cd5975250604064b0baed00bc7c201ea97c5e
Change-Id: Ida2cd5975250604064b0baed00bc7c201ea97c5e