Otapreopt: Implement new A/B OTA path

Refactor otapreopt_script, otapreopt_chroot and otapreopt such
that the actual work will be done as a child of the script driven
by update_engine.

The script now uses the new OtaDexoptService command to get the
right parameters for a dexopt call in otapreopt. As we reach
otapreopt_chroot and otapreopt directly, we can add parameters
without tainting the regular installd path. Use this to add the
target slot suffix, and make naming of the A/B artifacts include
the suffix instead of the generic "b."

Otapreopt_chroot startup is slightly complicated because all file
descriptors must be closed. Otherwise, dex2oat needs selinux
permissions to inherit the descriptors from update_engine.

Bug: 25612095
Bug: 28069686
Change-Id: I9d7b86ac8ecfdf91af3800f7e96f41534c0afc0f