DO NOT MERGE: Delete TestNetworkStackLib and reduce NetworkStack lib visibility.

TestNetworkStackLib is unused now that its callers have been
migrated to NetworkStackApi{Current,Stable}Lib. Remove it.

Also limit visibility of NetworkStackApi{Current,Stable}Lib,
since it's basically the entirety of networkstack code.

Test: m
Bug: 167645754
Ignore-AOSP-First: conflict with "enabled: false" in mainline-prod
Change-Id: Id0f5ef4c99d242c6a42853c3ca03fba41b14f535