Add sepolicy for /metadata/tradeinmode.

This defines a new type, tradeinmode_metadata_file, which is applied to
/metadata/tradeinmode. This directory contains an indicator, written by
system_server, that a factory reset must be initiated. The indicator is
read by first-stage init.

A neverallow rule is included since we don't want random processes reading or writing to this directory.

Bug: 307713521
Test: adb shell tradeinmode enter
Change-Id: Icc3c815a77bbadc1d4b32b88226e55a5595f5388
diff --git a/private/file.te b/private/file.te
index 662d5cc..60aa5d5 100644
--- a/private/file.te
+++ b/private/file.te
@@ -218,6 +218,8 @@
 # /data/misc_ce/userId/storage_area_keys
 type storage_area_key_file, file_type, data_file_type, core_data_file_type;
 
+# /metadata/tradeinmode files
+type tradeinmode_metadata_file, file_type;
 
 # Types added in 202504 in public/file.te
 until_board_api(202504, `