sepolicy: Switch to BOARD_VENDOR_SEPOLICY_DIRS

- BOARD_SEPOLICY_DIRS is deprecated and gives compile-time
  errors when used in unison with a device using BOARD_VENDOR_SEPOLICY_DIRS

Ref:
[0]: https://github.com/LineageOS/android_system_sepolicy/commit/ec3ac470a98342c13c1fec8d46433c73b08531be



Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>
Change-Id: Icefb062cc8cdef532b4310684d9a66afe97e49c4