Fix flaky tests in NetworkPolicyManagerServiceTest.

Test: runtest -c com.android.server.NetworkPolicyManagerServiceTest frameworks-services
Change-Id: I0a6f0d11e3922bfeb2bd3784f23e10ae45e5e40c
diff --git a/services/core/java/com/android/server/net/NetworkPolicyManagerService.java b/services/core/java/com/android/server/net/NetworkPolicyManagerService.java
index b165d34..f180c50 100644
--- a/services/core/java/com/android/server/net/NetworkPolicyManagerService.java
+++ b/services/core/java/com/android/server/net/NetworkPolicyManagerService.java
@@ -432,7 +432,8 @@
             mListeners = new RemoteCallbackList<>();
 
     final Handler mHandler;
-    final Handler mUidEventHandler;
+    @VisibleForTesting
+    public final Handler mUidEventHandler;
 
     private final ServiceThread mUidEventThread;