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/vr_flinger.cpp b/libs/vr/libvrflinger/vr_flinger.cpp
index 82ce067..9163e71 100644
--- a/libs/vr/libvrflinger/vr_flinger.cpp
+++ b/libs/vr/libvrflinger/vr_flinger.cpp
@@ -106,5 +106,13 @@
   }
 }
 
+void VrFlinger::OnHardwareComposerRefresh() {
+  if (display_service_) {
+    display_service_->OnHardwareComposerRefresh();
+  } else {
+    ALOGE("OnHardwareComposerRefresh failed : Display service is not started.");
+  }
+}
+
 }  // namespace dvr
 }  // namespace android