Call clearCallingIdentity when calling registerNetworkProvider in TNS

The caller to create a TestNetworkService via
ConnectivityManager#startOrGetTestNetworkManager should have
passed the MANAGE_TEST_NETWORKS permission check and the caller
doesn't need to declare the permission to pass the permission
check of registerNetworkProvider. So call clearCallingIdentity
when calling registerNetworkProvider in TestNetworkService.

Bug: 181573283
Test: atest CtsNetTestCases:ConnectivityManagerTest#testRequestBackgroundNetwork
Change-Id: Ia28627dacf933d1937978ed9709b975c9a4660ce
diff --git a/services/core/java/com/android/server/TestNetworkService.java b/services/core/java/com/android/server/TestNetworkService.java
index ee61067..f566277 100644
--- a/services/core/java/com/android/server/TestNetworkService.java
+++ b/services/core/java/com/android/server/TestNetworkService.java
@@ -90,7 +90,12 @@
         mCm = mContext.getSystemService(ConnectivityManager.class);
         mNetworkProvider = new NetworkProvider(mContext, mHandler.getLooper(),
                 TEST_NETWORK_PROVIDER_NAME);
-        mCm.registerNetworkProvider(mNetworkProvider);
+        final long token = Binder.clearCallingIdentity();
+        try {
+            mCm.registerNetworkProvider(mNetworkProvider);
+        } finally {
+            Binder.restoreCallingIdentity(token);
+        }
     }
 
     /**