Expose uids related APIs in NetworkRequest and NetworkCapabilities

NetworkRequest is moving into the incoming connectivity mainline
module. The hidden setUids becomes inaccessible outside the
module. Shims for support cts in different API levels will need
to use it to verify the behavior of NetworkRequest. Thus, expose
it to the API surface.

Also, VPN uses getUids and setUids to control network
capabilities. Networkcapabilities is a part of incoming
connectivity mainline module but VPN is not. Thus, exposing these
two methods are needed to allow VPN to continue using it.

Test: make update-api
Bug: 172183305
Merged-In: I107c329d4d7130d488772166eae8b5e7aaa2ff04
Change-Id: If76877e6bde7abe3b5ddde078f87eefd027e675c