Create proxy NetworkAgent for testing

This change adds a proxy NetworkAgent to facilitate testing, adding the
ability to test final methods such as sendNetworkCapabilities() and
sendLinkProperties(), which would otherwise call a real implementation,
and throw a null pointer while trying to access internal fields.

Bug: 184579891
Bug: 184697651
Test: atest FrameworksVcnTests
Change-Id: I8f13159bd266bf488f095a13d6ffe444bec65f38