Expose systemReady for SystemServer

SystemServer cannot call the hidden API of ConnectivityManager
after it becomes a part of mainline module. So expose the hidden
API for SystemServer.

Bug: 182963354
Test: m services
Change-Id: I9c1dc8eb1401dbdc069d5c9fc3992f4c7939b70e

framework/src/android/net/ConnectivityManager.java

@@ -915,8 +915,8 @@ public class ConnectivityManager {
 
     /**
      * @hide
-     * TODO: Expose for SystemServer when becomes a module.
      */
+    @SystemApi(client = MODULE_LIBRARIES)
     public void systemReady() {
         try {
             mService.systemReady();