init: load /product selinux mapping file

/system/etc/selinux/mapping/V.v.cil is being split into /system and
/product parts.

Change init to account for /product/etc/selinux/mapping/V.v.cil mapping
file when merging SELinux policies.

Bug: 119305624
Test: boot taimen
Test: "cnd" type is declared in /vendor; "dataservice_app" type is
declared in /product. This permission is preserved
"allow cnd dataservice_app:binder { transfer call };"
Change-Id: I65995a75041044ae71c148d655a70c5efd3b84cc