Make com.android.tethering the flag container instead of system

It was set to "system" incorrectly.

Bug: 312769710
Test: m all_aconfig_declarations
      printflags | grep android.ne
Change-Id: Ie65dcb8c987cc0677b80ba9f8f84fe604ebbf1d2
diff --git a/common/thread_flags.aconfig b/common/thread_flags.aconfig
index 43fc147..43acd1b 100644
--- a/common/thread_flags.aconfig
+++ b/common/thread_flags.aconfig
@@ -1,5 +1,5 @@
 package: "com.android.net.thread.flags"
-container: "system"
+container: "com.android.tethering"
 
 flag {
     name: "thread_enabled"