Otapreopt: Adapt for actual A/B

Add postinstall script for update_engine hook.

Add otapreopt_chroot as a gateway between installd and otapreopt.
Installd will fork and run otapreopt_chroot, which has the permission
to set up a chroot in /postinstall and run otapreopt from the B
partition.

Bug: 25612095
Change-Id: I4264598da00053ced87c849c738ddc0bc5437304