[Thread] flagging Android Thread APIs

This commit declares the initial "thread_enabled" feature flag and use
this to flag all existing and future Thread system APIs with @FlaggedApi

Bug: 301473012
Test: build
Change-Id: I43f64ce38f7fd0806733b21c4f393e4563e6a467
diff --git a/thread/flags/thread_base.aconfig b/thread/flags/thread_base.aconfig
new file mode 100644
index 0000000..bf1f288
--- /dev/null
+++ b/thread/flags/thread_base.aconfig
@@ -0,0 +1,8 @@
+package: "com.android.net.thread.flags"
+
+flag {
+    name: "thread_enabled"
+    namespace: "thread_network"
+    description: "Controls whether the Android Thread feature is enabled"
+    bug: "301473012"
+}