otapreopt: use boot image from ART APEX

Ensure presence of boot image and no longer try to generate if
missing. This is now an error.

Exempt-From-Owner-Approval: cherry-pick from rvc-dev

Bug: 149794809
Test: manual OTA
Change-Id: I565c24dad8bd10a93cd92b2926a2d8b45f416e41
Merged-In: I565c24dad8bd10a93cd92b2926a2d8b45f416e41
(cherry picked from commit e695e01b)