Create selinux_policy phony target

Moves selinux policy build decisions to system/sepolicy/Android.mk.
This is done because the PRODUCT_FULL_TREBLE variable isn't available
in embedded.mk and TARGET_SANITIZE isn't available to dependencies of
init.

Test: Build/boot Bullhead PRODUCT_FULL_TREBLE=false
Test: Build/boot Marlin PRODUCT_FULL_TREBLE=true
Test: Build Marlin TARGET_SANITIZE=address. Verify asan rules are
      included in policy output.
Bug: 36138508
Change-Id: I20a25ffdfbe2b28e7e0f3e090a4df321e85e1235