Add flag for HSUM Telecom fixes.

This CL adds a flag to use for adding support for HSUM in Telecom.

Test: ExtendedInCallServiceTest#testOnCannedTextResponsesLoaded
Flag: com.android.server.telecom.flags.telecom_main_user_in_get_respond_message_app
Bug: 358587742
Change-Id: I13babeb2b43cf202d3e4decf8043efaae6843377
diff --git a/flags/Android.bp b/flags/Android.bp
index e374a4d..501eba4 100644
--- a/flags/Android.bp
+++ b/flags/Android.bp
@@ -43,6 +43,7 @@
         "telecom_profile_user_flags.aconfig",
         "telecom_bluetoothdevicemanager_flags.aconfig",
         "telecom_non_critical_security_flags.aconfig",
+        "telecom_headless_system_user_mode.aconfig",
         "telecom_metrics_flags.aconfig",
     ],
 }