sepolicy for vendor overlay

Make /(product|system/product)/vendor_overlay/<ver> have the vendor
file context.

If vendor_overlay requires to mount on the vendor directories other
than 'vendor_file', the contexts must be defined in the device
specific sepolicy files.

Bug: 119076200
Test: build and check if the files are overided and have the required
sepolicy contexts.

Change-Id: I69ed38d4ea8e7d89f56865b1ca1e26f290e9892d

private/file_contexts

@@ -381,6 +381,14 @@
 /(product_services|system/product_services)(/.*)?               u:object_r:system_file:s0
 /(product_services|system/product_services)/overlay(/.*)?       u:object_r:vendor_overlay_file:s0
 
+#############################
+# Vendor files from /(product|system/product)/vendor_overlay
+#
+# NOTE: For additional vendor file contexts for vendor overlay files,
+# use device specific file_contexts.
+#
+/(product|system/product)/vendor_overlay/[0-9]+/.*          u:object_r:vendor_file:s0
+
 #############################
 # Data files
 #