Give mount & chroot permissions to otapreopt_chroot related to
postinstall.

Add postinstall_dexopt for otapreopt in the B partition. Allow
the things installd can do for dexopt. Give a few more rights
to dex2oat for postinstall files.

Allow postinstall files to call the system server.

Bug: 25612095
Change-Id: If7407473d50c9414668ff6ef869c2aadd14264e7