Fix PolicyListener testing surface.

This CL updates VcnManager to provide a test-only method for getting
currently registered VcnUnderlyingNetworkPolicyListeners. This is
preferrable over the current method, which accesses VcnManager's private
Map directly.

Bug: 8675309
Test: atest FrameworksVcnTests
Change-Id: I7afd60ff55868ae44bfa74910ae02daf8a81b107