[Thread] define flag thread_user_restriction_enabled

Define this flag for guarding UserManager#DISALLOW_THREAD_NETWORK

Bug: 307679182
Change-Id: I69c578d76f3aac250bb733aac74754c127ae453b
diff --git a/thread/flags/thread_base.aconfig b/thread/flags/thread_base.aconfig
index bf1f288..f73ea6b 100644
--- a/thread/flags/thread_base.aconfig
+++ b/thread/flags/thread_base.aconfig
@@ -6,3 +6,10 @@
     description: "Controls whether the Android Thread feature is enabled"
     bug: "301473012"
 }
+
+flag {
+    name: "thread_user_restriction_enabled"
+    namespace: "thread_network"
+    description: "Controls whether user restriction on thread networks is enabled"
+    bug: "307679182"
+}