Allow to specify platform sepolicy dir mult. times

For automotive (and I assume for other verticals) it make sense to keep
vertical-specific policies outside of /system/sepolicy as those not used
by the phones. However, there's no way to do it rather than using
BOARD_PLAT_{PUBLIC|PRIVATE}_SEPOLICY_DIR build variables.

Bug: 70637118
Test: lunch bat_land-userdebug && m
Test: verify it builds, boots and logs seems to be reasonable
Test: enable full treble for aosp_car_x86 - verify it builds, boots and
no denials in the logs

Change-Id: Ia5fd847f7a6152ff6cf99bbbc12e1e322f7946ab