Make TranslucentConversionListener a SystemApi

Change-Id: I59a93e1e040fdbaa9ef108b31e08e4553ba82ae1
diff --git a/core/java/android/app/Activity.java b/core/java/android/app/Activity.java
index 9bbd544..21c8b75 100644
--- a/core/java/android/app/Activity.java
+++ b/core/java/android/app/Activity.java
@@ -6119,6 +6119,7 @@
      *
      * @hide
      */
+    @SystemApi
     public interface TranslucentConversionListener {
         /**
          * Callback made following {@link Activity#convertToTranslucent} once all visible Activities