Adds a skeleton component app for Always-On Hotwording opt-in flow.

chrome/common/extensions/extension_constants.h

Index: chrome/common/extensions/extension_constants.h
diff --git a/chrome/common/extensions/extension_constants.h b/chrome/common/extensions/extension_constants.h
index 4bbffb39236fd2ba78776363e822fce8de16b626..880ac1a059bc496b78a413f7dc7792ee84c03df5 100644
--- a/chrome/common/extensions/extension_constants.h
+++ b/chrome/common/extensions/extension_constants.h
@@ -116,6 +116,9 @@ extern const char kIdentityApiUiAppId[];
 // The extension id of the Crosh component app for ChromeOS.
 extern const char kCroshBuiltinAppId[];
 
+// The extension id of the hotword audio verification dialogue app.
+extern const char kHotwordAudioVerificationAppId[];
+
 // The extension id of the hotword voice search trigger extension.
 extern const char kHotwordExtensionId[];