Set the container field of aconfig flags

Test: m
Bug: 312769710
Change-Id: I1eed7982b40d28947aa3f9575c568eac325dc832
diff --git a/thread/flags/thread_base.aconfig b/thread/flags/thread_base.aconfig
index bf1f288..09595a6 100644
--- a/thread/flags/thread_base.aconfig
+++ b/thread/flags/thread_base.aconfig
@@ -1,4 +1,5 @@
 package: "com.android.net.thread.flags"
+container: "system"
 
 flag {
     name: "thread_enabled"