Reorganize NetworkStack common srcs

This specifies the exact set of files in NetworkStack that are used in
frameworks/base/services/net. It will allow us to add other files to the
directory without exposing them to services/net (which would make it
harder to remove all services.net dependencies on NetworkStack).

Test: m
Change-Id: I6abee79523d04c0c3f9ed3e96b1dd9cb37dc4cc3