Renames nonplat_* to vendor_*

This change renames the non-platform sepolicy files on a DUT from
nonplat_* to vendor_*.

It also splits the versioned platform sepolicy from vendor_sepolicy.cil
to a new file /vendor/etc/selinux/plat_pub_versioned.cil. And only keeps
vendor customizations in vendor_sepolicy.cil.

Build variable BOARD_SEPOLICY_DIRS is also renamed to
BOARD_VENDOR_SEPOLICY_DIRS.

Bug: 64240127
Test: boot bullhead/taimen
Change-Id: Iea2210c9c8ab30c9ecbcd8146f074e76e90e6943