Sepolicy: Give Otapreopt right to write A/B artifacts in /data

For apps installed /data (vs the system image), Otapreopt puts
A/B artifacts alongside the regular oat location with a "b" suffix.

Give Otapreopt the right to create and write such files.

Bug: 25612095
Change-Id: Idf3f0959816f49407036cea9b8e684b26c510e80