Add /product sepolicy support to SELinuxMMAC.java

Bug: 119305624
Test: normal/recovery boot aosp_taimen
Test: this log entry is in logcat
SELinuxMMAC: Using policy file /product/etc/selinux/product_mac_permissions.xml
Test: wfcactivation app works without denials. Its mac permissions are
now in /product.
Change-Id: I977ad0d763e46dbcb5bff36fc6361b4e70098c13