reduce log level when no interface is present for rate limiting

TestNetworkAgentWrapper does not set the interface name in its
LinkProperties causing logwtf to fail tests on -eng build.

Test: atest FrameworksNetTests:ConnectivityServiceTest on -eng build
Change-Id: Ieba0453ce897aa1052cb98f1de4c7bb099383c8d
diff --git a/service/src/com/android/server/ConnectivityService.java b/service/src/com/android/server/ConnectivityService.java
index 6024a2a..271dd17 100644
--- a/service/src/com/android/server/ConnectivityService.java
+++ b/service/src/com/android/server/ConnectivityService.java
@@ -10691,8 +10691,9 @@
 
         final String iface = networkAgent.linkProperties.getInterfaceName();
         if (iface == null) {
-            // This can never happen.
-            logwtf("canNetworkBeRateLimited: LinkProperties#getInterfaceName returns null");
+            // This may happen in tests, but if there is no interface then there is nothing that
+            // can be rate limited.
+            loge("canNetworkBeRateLimited: LinkProperties#getInterfaceName returns null");
             return false;
         }
         return true;