system_api: Public InputNodeGainChanged for dbus api

InputNodeGainChanged indicate the current gain for the node

BUG=b:255278344
TEST=cros-workon-${BOARD} start chromeos-base/system_api
TEST=FEATURES=test emerge-${BOARD} chromeos-base/system_api

Cq-Depend: chromium:4125095
Change-Id: I9f1121996d3addf674ca197741a487eeb560235d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/4125078
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
Reviewed-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Commit-Queue: Baili Deng <bailideng@google.com>
Tested-by: Baili Deng <bailideng@google.com>
NOKEYCHECK=True
GitOrigin-RevId: b764b9d29ed2420194badca532c4e0feded2b334
diff --git a/dbus/audio/dbus-constants.h b/dbus/audio/dbus-constants.h
index 0357ce7..11d04e7 100644
--- a/dbus/audio/dbus-constants.h
+++ b/dbus/audio/dbus-constants.h
@@ -99,6 +99,7 @@
 const char kNodeLeftRightSwappedChanged[] = "NodeLeftRightSwappedChanged";
 const char kInputGainChanged[] = "InputGainChanged";
 const char kInputMuteChanged[] = "InputMuteChanged";
+const char kInputNodeGainChanged[] = "InputNodeGainChanged";
 const char kNodesChanged[] = "NodesChanged";
 const char kActiveOutputNodeChanged[] = "ActiveOutputNodeChanged";
 const char kActiveInputNodeChanged[] = "ActiveInputNodeChanged";