Add missing container field to the aconfig flag file.

Bug: 328823994
Test: CI
Change-Id: I40e50c1c8571c1c88e7f8f582cf4e1155af713fd
diff --git a/aconfig/settings_threadnetwork_flag_declarations.aconfig b/aconfig/settings_threadnetwork_flag_declarations.aconfig
index 70945d3..e5448a9 100644
--- a/aconfig/settings_threadnetwork_flag_declarations.aconfig
+++ b/aconfig/settings_threadnetwork_flag_declarations.aconfig
@@ -1,4 +1,5 @@
 package: "com.android.settings.flags"
+container: "system"
 
 flag {
     name: "thread_settings_enabled"