Rename plat_pub_policy -> pub_policy

Public policy that is available to vendor (and odm) sepolicy is a
combination of system and product public sepolicy. Since "plat_" prefix
implies a pure system sepolicy component, drop "plat_" prefix from
"plat_pub_policy" to be consistent with naming in this file.

Bug: 119305624
Test: m selinux_policy
Change-Id: Iaf094702556ce97371fa1c58c01d707103d7f7d6