Include correct type of SELinux policy

This makes the build system include split SELinux policy (three CIL
files and the secilc compiler needed to compile them) if
PRODUCT_FULL_TREBLE is set to true. Otherwise, the monolitic SELinux
policy is included.

Split policy currently adds around 400 ms to boot time (measured on
marlin/sailfish and bullhead) because the policy needs to be compiled
during boot. This is the main reason why we include split policy only
on devices which require it.

Test: Device boots, no additional SELinux denials. This test is
      performed on a device with PRODUCT_FULL_TREBLE set to true, and
      on a device with PRODUCT_FULL_TREBLE set to false.
Test: Device with PRODUCT_FULL_TREBLE set to true contains secilc and
      the three *.cil files, but does not contain the sepolicy file.
      Device with PRODUCT_FULL_TREBLE set to false contains sepolicy
      file but does not contain the secilc file or any *.cil files.
Bug: 31363362

Change-Id: I419aa35bad6efbc7f936bddbdc776de5633846fc