Track default upstream when system is ready

Start tracking default upstream from boot.This is useful for
entitlement refine in following change. EntitlementManager can
decide if it needs to process entitlement provisioning before
tethering started.

Test: -atest FrameworksNetTests
      -build, flash, booted
      -manually turnoff/on tethering with different upstream
bug: 111490073

Change-Id: I8fdbd64c52f26b5363693bb5bd8050930e8ea961

services/core/java/com/android/server/ConnectivityService.java

@@ -1968,6 +1968,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
     void systemReady() {
         mProxyTracker.loadGlobalProxy();
         registerNetdEventCallback();
+        mTethering.systemReady();
 
         synchronized (this) {
             mSystemReady = true;