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/hardware_composer.cpp b/libs/vr/libvrflinger/hardware_composer.cpp
index 5253b26..e6ed665 100644
--- a/libs/vr/libvrflinger/hardware_composer.cpp
+++ b/libs/vr/libvrflinger/hardware_composer.cpp
@@ -1287,6 +1287,10 @@
   // TODO(eieio): implement display hotplug callbacks.
 }
 
+void HardwareComposer::OnHardwareComposerRefresh() {
+  // TODO(steventhomas): Handle refresh.
+}
+
 void HardwareComposer::SetBacklightBrightness(int brightness) {
   if (backlight_brightness_fd_) {
     std::array<char, 32> text;