sepolicy: fix missing label on vendor_service_contexts

Vendors can publish services with servicemanager only on non-Treble
builds. vendor_service_contexts is not meant to be read by
servicemanager.

https://android.googlesource.com/platform/system/sepolicy/+/5bccbfefe494aa6fed73a58d80713e28257ba7f1/public/servicemanager.te#22

Bug: 141333155
Test: create /vendor/etc/selinux/vendor_service_contexts and make sure it is
correctly labeled.
Change-Id: Ib68c50e0cdb2c39f0857a10289bfa26fa11b1b3c
diff --git a/private/file_contexts b/private/file_contexts
index a552d47..52354e2 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -363,6 +363,8 @@
 # HAL location
 /(vendor|system/vendor)/lib(64)?/hw            u:object_r:vendor_hal_file:s0
 
+/(vendor|system/vendor)/etc/selinux/(vendor|nonplat)_service_contexts u:object_r:nonplat_service_contexts_file:s0
+
 #############################
 # OEM and ODM files
 #