[Thread] make ThreadNetworkController#setConfiguration a system API

This CL turns setConfiguration into a system API, including the
ThreadConfiguration#Builder
and the #setNat64Enabled method.

Bug: 368456504

Change-Id: I0ff2ed81271dcceea295d169a46a02f05aeda5d6
diff --git a/common/thread_flags.aconfig b/common/thread_flags.aconfig
index 14b70d0..60120bc 100644
--- a/common/thread_flags.aconfig
+++ b/common/thread_flags.aconfig
@@ -35,3 +35,12 @@
     description: "Controls whether the Android Thread Ephemeral Key feature is enabled"
     bug: "348323500"
 }
+
+flag {
+    name: "set_nat64_configuration_enabled"
+    is_exported: true
+    is_fixed_read_only: true
+    namespace: "thread_network"
+    description: "Controls whether the setConfiguration API of NAT64 feature is enabled"
+    bug: "368456504"
+}