selinux: account for /product hash

We need to be able to tell if /product was updated independently
/system.  Otherwise, the device might accidentally load the
precompiled_policy after a /product sepolicy update.

Bug: 119305624
Test: boot aosp_taimen, precompiled policy is loaded
Test: If either of these hashes
/system/etc/selinux/plat_sepolicy_and_mapping.sha256
/product/etc/selinux/product_sepolicy_and_mapping.sha256
are removed, then init falls back to compiling sepolicy at boot time.
Change-Id: I3d658335cf66bcb7123109de701309039b1ae5ba