Add NetworkShim

NetId is exposed as system API after API 29. NetworkStack will
need netId to get the fwmark information from netd. Because
NetworkStack will need at least two different build to support
the API usage. Thus, create NetworkShim to support design
compatible.

Bug: 130325409
Test: atest NetworkStackTests NetworkStackNextTests
Change-Id: I1b8fa69b4c0fece95a81e4aa43ef84dc68852237