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

framework/src/android/net/ConnectivityManager.java

@@ -1192,7 +1192,8 @@ public class ConnectivityManager {
      *
      * @return a {@link Network} object for the current default network for the
      *         given UID or {@code null} if no default network is currently active
-     * TODO: b/183465229 Cleanup getActiveNetworkForUid once b/165835257 is fixed
+     *
+     * @hide
      */
     @RequiresPermission(android.Manifest.permission.NETWORK_STACK)
     @Nullable