Support NetworkRequest.Builder with an existing request in shim

New constructor in NetworkRequest.Builder that takes an existing
network request is exposed for supporting the usage outside the
module. This API should be tested. Add the new method into shims
to allow building and testing against older sdk version without
this new API.

Bug: 172183305
Test: make NetworkStackApiStableLib NetworkStackApiCurrentLib
Change-Id: I61227e5dc7d48b1ff71d52b5ec9d9644fe33ffab