Use Context#getSystemService to get network stack binder

Bug: 143195885
Test: -build, flash, boot
      -atest TetheringTests

Change-Id: Ic5fabeae27677344d691449c31ad9337c0e6d92c
diff --git a/Tethering/src/com/android/server/connectivity/tethering/TetheringService.java b/Tethering/src/com/android/server/connectivity/tethering/TetheringService.java
index e4e4a09..99130d2 100644
--- a/Tethering/src/com/android/server/connectivity/tethering/TetheringService.java
+++ b/Tethering/src/com/android/server/connectivity/tethering/TetheringService.java
@@ -42,7 +42,6 @@
 import android.os.Looper;
 import android.os.RemoteException;
 import android.os.ResultReceiver;
-import android.os.ServiceManager;
 import android.os.SystemProperties;
 import android.os.UserManager;
 import android.provider.Settings;
@@ -363,7 +362,7 @@
                     IBinder connector;
                     try {
                         final long before = System.currentTimeMillis();
-                        while ((connector = ServiceManager.getService(
+                        while ((connector = (IBinder) mContext.getSystemService(
                                 Context.NETWORK_STACK_SERVICE)) == null) {
                             if (System.currentTimeMillis() - before > NETWORKSTACK_TIMEOUT_MS) {
                                 Log.wtf(TAG, "Timeout, fail to get INetworkStackConnector");