Separate system_ext_file_contexts out of system sepolicy.

Bug: 137712473
Test: boot crosshatch
Change-Id: I09f63771d08ad18fb41fca801dd587b086be58c7
diff --git a/private/file_contexts b/private/file_contexts
index 1e9549c..8d4e6bc 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -422,6 +422,8 @@
 /(system_ext|system/system_ext)/etc/passwd          u:object_r:system_passwd_file:s0
 /(system_ext|system/system_ext)/overlay(/.*)?       u:object_r:vendor_overlay_file:s0
 
+/(system_ext|system/system_ext)/etc/selinux/system_ext_file_contexts  u:object_r:file_contexts_file:s0
+
 #############################
 # Vendor files from /(product|system/product)/vendor_overlay
 #