Run isKeyguardSecure under system privileges

In certain cases, isKeyguardSecure calls UserManager.getProfileParent, which
requires MANAGE_USERS permission.
Now the check is done under system privileges.

Bug:18765066
Change-Id: I6b23aedee06403d36523af5fee9c7db9659284b3

services/core/java/com/android/server/wm/WindowManagerService.java

@@ -5379,7 +5379,12 @@ public class WindowManagerService extends IWindowManager.Stub
 
     @Override
     public boolean isKeyguardSecure() {
-        return mPolicy.isKeyguardSecure();
+        long origId = Binder.clearCallingIdentity();
+        try {
+            return mPolicy.isKeyguardSecure();
+        } finally {
+            Binder.restoreCallingIdentity(origId);
+        }
     }
 
     @Override