Update dexpreopt_test to verify boot images.

This ensures that odrefresh does not unexpectedly override the boot
image on /system on the first boot.

Bug: 203492478
Test: Build a system image, flash it to a device, and run
  `atest art_standalone_dexpreopt_test`.
Change-Id: Id02e60612e6b3c12876cc8707e6d2be3866a5b6f
4 files changed