Add BOARD_PLAT_[PUBLIC|PRIVATE]_SEPOLICY_DIRS

Add new build variables for partner customization (additions) to platform sepolicy.
This allows partners to add their own policy without having to touch the AOSP sepolicy
directories and potentially disrupting compatibility with an AOSP system image.

Bug: 36467375
Test: Add public and private files to sailfish policy and verify that they are
added to the appropriate policy files, but that the policy is otherwise identical.
Also add private/mapping/*.cil files in both locations and change the BOARD_SEPOLICY_VERS
to trigger use of prebuilt mapping files and verify that they are appropriately
combined and built in policy.

Change-Id: I38efe2248520804a123603bb050bba75563fe45c
Merged-In: I38efe2248520804a123603bb050bba75563fe45c
(cherry picked from commit f893700c)