Add tests for setUnderlyingNetworks.

Fixes come later. This is complex enough as it is.

Clean cherry-pick of ag/4083953

Bug: 79748782
Test: new test passes, old tests still pass
Change-Id: If7276fe1f751be7b9c18f689e97699e566e5bde0
Merged-In: I12c948ebeb2b74290908f8320ff77220dc4a9fb9