ISpatializer: add missing @nullable annotation am: 2abe519146 am: 551ce62df0

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/av/+/15852971

Change-Id: I9ba566a5c22193ae145433e6d59466807a567947
diff --git a/media/libaudioclient/aidl/android/media/ISpatializer.aidl b/media/libaudioclient/aidl/android/media/ISpatializer.aidl
index 72e8b2e..6355b46 100644
--- a/media/libaudioclient/aidl/android/media/ISpatializer.aidl
+++ b/media/libaudioclient/aidl/android/media/ISpatializer.aidl
@@ -111,6 +111,6 @@
      * The last callback registered wins and passing a nullptr unregisters
      * last registered callback.
      */
-    void registerHeadTrackingCallback(ISpatializerHeadTrackingCallback callback);
+    void registerHeadTrackingCallback(@nullable ISpatializerHeadTrackingCallback callback);
 
 }