Remove unnecessary public API getActiveNetworkForUid

We have exposed the CS module API getActiveNetworkForUid
(Used in Jobscheduler) as a temporary solution for the mainline
Connectivity service module. This API usage has now been removed
in Jobscheduler(b/165835257), so we can permanently remove the
public getActiveNetworkForUid API.

Bug: 183465229
Test: atest FrameworksNetTests
Change-Id: I0f23272d2d8a5118f2e11ac325fdb9e8f9dbe0e3