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_contexts b/private/file_contexts
index f837b70..f605e66 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -889,6 +889,7 @@
 /metadata/aconfig(/.*)?    u:object_r:aconfig_storage_metadata_file:s0
 /metadata/aconfig/flags(/.*)?    u:object_r:aconfig_storage_flags_metadata_file:s0
 /metadata/aconfig_test_missions(/.*)?    u:object_r:aconfig_test_mission_files:s0
+/metadata/tradeinmode(/.*)?    u:object_r:tradeinmode_metadata_file:s0
 
 ############################
 # mount point for ota metadata