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.

Test: atest FrameworksNetTests NetworkStackTests
Bug: 124033493
Change-Id: Ib1d08a3669983640119d008db7e2990fa798724f
Merged-In: I501b125a388c1100c2182bde4670944c2f0d7a02