Add rules for /product{,_services}/overlay

These rules mirror those for /vendor/overlay and /odm/overlay, including
the possilibity of a symlink like /system/vendor -> /vendor.

Test: builds, boots
Change-Id: I323e48fcc13c4ac7779902506539c2600708cc88

private/file_contexts

@@ -376,11 +376,13 @@
 # Product files
 #
 /(product|system/product)(/.*)?                                 u:object_r:system_file:s0
+/(product|system/product)/overlay(/.*)?                         u:object_r:vendor_overlay_file:s0
 
 #############################
 # Product-Services files
 #
 /(product_services|system/product_services)(/.*)?               u:object_r:system_file:s0
+/(product_services|system/product_services)/overlay(/.*)?       u:object_r:vendor_overlay_file:s0
 
 #############################
 # Data files