Move NetworkStack to services.net

NetworkStack is only used in services.net or clients of services.net. It
cannot stay in framework.jar because it needs to depend on AIDL
interfaces, which would conflict with app implementations if they were
in framework.jar.

(cherry-pick of aosp/905233 with trivial conflicts in SystemServer.java)

Test: atest FrameworksNetTests NetworkStackTests
Bug: 124033493
Change-Id: I501b125a388c1100c2182bde4670944c2f0d7a02