Move otadexopt-related logic to otapreopt_chroot binary

This allows us to only mount APEXes once during postinstall, instead of
mounting them for each APK being optimized.

Ultimately this CL just moves the while loop from otapreopt_script.sh
into otapreopt_chroot binary, and makes otapreopt_chroot communicate
with otadexopt service via binder.

Without this patch a no-op OTA takes 669.617 seconds
With this patch: 361.991 seconds

Bug: 190817237
Test: manual OTA
Change-Id: I02e80a8e68b6d274a007599371e43cd15330d351