Include selinux_policy_* for product and system_ext.

After separating selinux_policy_system_ext and selinux_policy_product
from the selinux_policy_nonsystem module, add them to
base_system_ext.mk and base_product.mk, respectively.

Bug: 170282998
Test: build and check boot
Change-Id: I03a4ae435898dd773f418d37f321ad0a37f17c3e