Add a default rule for /product files

Since /product is an extension of /system, its file contexts should be
consistent with ones of /system.

Bug: 64195575
Test: tested installing a RRO, apps, priv-apps and permissions
Change-Id: I7560aaaed852ba07ebe1eb23b303301481c897f2

private/file_contexts

@@ -324,6 +324,11 @@
 /oem(/.*)?              u:object_r:oemfs:s0
 
 
+#############################
+# Product files
+#
+/(product|system/product)(/.*)?               u:object_r:system_file:s0
+
 #############################
 # Data files
 #