Ignore callbacks from the non-active hardware composer

Ignore callbacks from the non-active hardware composer so we don't get
e.g. duplicate vsync callbacks, one from each composer.

Bug: None

Test: Manually confirmed with logs we're now ignoring callbacks on the
      non-active composer.

Change-Id: I8b475d6283d86c64ff96b41e78528bce8c6ff1d3
diff --git a/libs/vr/libvrflinger/display_service.h b/libs/vr/libvrflinger/display_service.h
index ebd97de..5de4f1d 100644
--- a/libs/vr/libvrflinger/display_service.h
+++ b/libs/vr/libvrflinger/display_service.h
@@ -68,6 +68,8 @@
     }
   }
 
+  void OnHardwareComposerRefresh();
+
  private:
   friend BASE;
   friend DisplaySurface;