Spatializer: Add head tracking callback

Split head tracking related events from main INativeSpatializerCallback
to a dedicated callback ISpatializerHeadTrackingCallback that can
be registerred and unregistered independently.

Bug: 188502620
Test: manual test with mock spatializer
Change-Id: I09eefb0911f557a6faa6cf0b6c1afe7eaf71ca84
diff --git a/media/libaudioclient/Android.bp b/media/libaudioclient/Android.bp
index fbbd7ff..3c3d340 100644
--- a/media/libaudioclient/Android.bp
+++ b/media/libaudioclient/Android.bp
@@ -502,6 +502,7 @@
     srcs: [
         "aidl/android/media/INativeSpatializerCallback.aidl",
         "aidl/android/media/ISpatializer.aidl",
+        "aidl/android/media/ISpatializerHeadTrackingCallback.aidl",
     ],
     imports: [
         "audiopolicy-types-aidl",