Reland: Add system_ext_userdebug_plat_sepolicy.cil for GSI

system_ext_userdebug_plat_sepolicy.cil is a copy of
userdebug_plat_sepolicy.cil (debug_ramdisk) that's installed in the
system_ext partition.
The build rule is gated by a BoardConfig variable, so products other
than GSI cannot accidentally install this module.

Bug: 188067818
Test: Flash RQ2A.201207.001 bramble-user with debug ramdisk & flash
  gsi_arm64-user from master, device can boot and `adb root` works
Change-Id: I75183e2dfdb434aee0b015b1627c9e23b4f3437f
diff --git a/private/file_contexts b/private/file_contexts
index 8849602..ed972c0 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -477,6 +477,7 @@
 /(system_ext|system/system_ext)/etc/selinux/system_ext_seapp_contexts       u:object_r:seapp_contexts_file:s0
 /(system_ext|system/system_ext)/etc/selinux/system_ext_service_contexts     u:object_r:service_contexts_file:s0
 /(system_ext|system/system_ext)/etc/selinux/system_ext_mac_permissions\.xml u:object_r:mac_perms_file:s0
+/(system_ext|system/system_ext)/etc/selinux/userdebug_plat_sepolicy\.cil    u:object_r:sepolicy_file:s0
 
 /(system_ext|system/system_ext)/bin/aidl_lazy_test_server    u:object_r:aidl_lazy_test_server_exec:s0
 /(system_ext|system/system_ext)/bin/aidl_lazy_cb_test_server u:object_r:aidl_lazy_test_server_exec:s0