[Thread] use dedicated flag for Thread settings

a larger THREAD_ENABLED_PLATFORM flag which guards all Thread-related
platform changes is used for the Thread settings change. But we need
more flexible control of the UX release timeline. So use a dedicated
THREAD_SETTINGS_ENABLED flag here.

Bug: 329384658
Change-Id: Ib5b3ae3c71ccc890fc46af982b021858774938f3

aconfig/settings_threadnetwork_flag_declarations.aconfig

@@ -0,0 +1,8 @@
+package: "com.android.settings.flags"
+
+flag {
+    name: "thread_settings_enabled"
+    namespace: "thread_network"
+    description: "Controls whether the Thread Settings UX is displayed"
+    bug: "329384658"
+}