Targets for artifacts needed by recovery

This ensures that SELinux policy artifact needed by recovery at
runtime have targets in this build script. This is to make
recoveryimage/bootimage targets depend on these artifacts explicitly,
which reduces the element of surprise. Moreover, this enables us to
move non-recovery artifacts around without affecting recovery
artifacts.

Test: Clean build, flash, device boots just fine, no new denials.
      Reboot to recovery, recovery boots just fine, no denials.
      This was tested on bullhead (non A/B device) and sailfish (A/B
      device).
Bug: 33642277
Change-Id: I3c494d9d7fec5c4f487d38964e572757fcf67f57