Sepolicy: Adapt for new A/B OTA flow

To include target slot names in the naming of A/B OTA artifacts,
and new path has been implemented. Instead of passing through
the system server and forking off of installd, otapreopt_chroot
is now driven directly from the otapreopt script.

Change the selinux policy accordingly: allow a transition from
postinstall to otapreopt_chroot, and let otapreopt_chroot inherit
the file descriptors that update_engine had opened (it will close
them immediately, do not give rights to the downstream executables
otapreopt and dex2oat).

Bug: 25612095
Bug: 28069686
Change-Id: I6b476183572c85e75eda4d52f60e4eb5d8f48dbb