Restrict BOARD_PLAT_[PUBLIC|PRIVATE]_SEPOLICY_DIRS to one dir.

These directories were added to allow for partner extensions to the
android framework without needing to add changes to the AOSP global
sepolicy.  There should only ever be one owner of the framework and
corresponding updates, so enforce this restriction to prevent
accidental accrual of policy in the system image.

Bug: 36467375
Test: Add public and private files to policy and verify that they are
added to the appropriate policy files.  Also test that specifying
multiple directories for public or private results in an error.

Change-Id: I397ca4e7d6c8233d1aefb2a23e7b44315052678f