Specify intermediates dir for sepolicy

Policy intermediates are being placed in a seemingly random
intermediates directories.

Currently:
out/target/product/marlin/obj_arm/SHARED_LIBRARIES/libsoftkeymaster_intermediates

Instead, place intermediates in the sepolicy_intermediates dir.

Test: intermediates now placed in:
out/target/product/marlin/obj/ETC/sepolicy_intermediates
Test: Marlin builds, no change to sepolicy on device.
Bug: 36269118

Change-Id: Ib6e9d9033be4dc8db0cc66cb47d9dc35d38703fe