[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/framework-t/api/system-current.txt b/framework-t/api/system-current.txt
index f6b5657..06d3238 100644
--- a/framework-t/api/system-current.txt
+++ b/framework-t/api/system-current.txt
@@ -417,12 +417,12 @@
 
 package android.net.thread {
 
-  public class ThreadNetworkController {
+  @FlaggedApi("com.android.net.thread.flags.thread_enabled") public class ThreadNetworkController {
     method public int getThreadVersion();
     field public static final int THREAD_VERSION_1_3 = 4; // 0x4
   }
 
-  public class ThreadNetworkManager {
+  @FlaggedApi("com.android.net.thread.flags.thread_enabled") public class ThreadNetworkManager {
     method @NonNull public java.util.List<android.net.thread.ThreadNetworkController> getAllThreadNetworkControllers();
   }