Make image oat checksum depend on secondary images.

Rather than explicitly having to load and combine the checksums from
secondary images in the case of multi-image, have the image oat
checksum for the primary image depend on the contents of all the
images.

Bug: 35659889
Bug: 34385298
Bug: 35992406

Test: test-art-host
Test: Manually add field to ZygoteInit, update boot image, verify image
      is properly relocated and used on device.
Change-Id: I38bd957d165682edabd0fd1874e8ef7acf923deb