Remove IoUtils from service-permission system-server stubs

As this did not have @hide this was being treated as if it was part of
the public API and so while it was not being tracked in a .txt file
because only the system-server API was being generated it was being
included in the system-server stubs as metalava will include public API
stubs in the stubs for every API surface.

Adding @hide ensures that it is no longer exported as part of the
service-permission stubs.

Bug: 155164730
Test: m service-permission-stubs and verify that it does not include IoUtils
Change-Id: I1ffa284c1f692397b52bdd6b3750b13125887352

apex/permission/service/java/com/android/permission/persistence/IoUtils.java

@@ -20,6 +20,8 @@ import android.annotation.NonNull;
 
 /**
  * Utility class for IO.
+ *
+ * @hide
  */
 public class IoUtils {