[Thread] move Thread settings data to APEX data dir

This commit includes two sepolicy changes:
1. change threadnetwork data file to
/data/misc/apexdata/com.android.tethering/threadnetwork
2. use apex_tethering_data_file for files under
   /data/misc/apexdata/com.android.tethering

The background is that the Thread daemon (ot_daemon) is merged into the
Tethering mainline module, which means the the Tehtering module now has
code running in both system_server and the standalone unprivileged
ot_daemon process. To prevent ot_daemon from accessing other
apex_system_server_data_file dirs, here use the specific
apex_tethering_data_file for both Tethering and Thread files (A
subdirectory threadnetwork/ will be created for Thread at runtime). This
is similar to apex_art_data_file and apex_virt_data_file.

Note that a file_contexts rule like
```
/data/misc/apexdata/com\.android\.tethering/threadnetwork(/.*)?  u:object_r:apex_threadnetwork_data_file:s0
```
won't work because the threadnetwork/ subdir doesn't exist before the
sepolicy rules are evaluated.

Bug: 309932508
Test: manually verified that Thread settings file can be written to
      /data/misc/apexdata/com.android.tethering/threadnetwork
Change-Id: I66539865ef388115c8e9b388b43291d8faf1f384
diff --git a/private/system_server.te b/private/system_server.te
index 62430b1..decabe1 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -1418,19 +1418,19 @@
 # These are modules where the code runs in system_server, so we need full access.
 allow system_server apex_system_server_data_file:dir create_dir_perms;
 allow system_server apex_system_server_data_file:file create_file_perms;
+allow system_server apex_tethering_data_file:dir create_dir_perms;
+allow system_server apex_tethering_data_file:file create_file_perms;
 # Legacy labels that we still need to support (b/217581286)
 allow system_server {
   apex_appsearch_data_file
   apex_permission_data_file
   apex_scheduling_data_file
-  apex_tethering_data_file
   apex_wifi_data_file
 }:dir create_dir_perms;
 allow system_server {
   apex_appsearch_data_file
   apex_permission_data_file
   apex_scheduling_data_file
-  apex_tethering_data_file
   apex_wifi_data_file
 }:file create_file_perms;