Clean up OatFileAssistant getting of image info.

* ImageInfo.oat_checksum is no longer unused.
* Don't abort if we can't read secondary isa images.

Test: m art-test-host
Bug: 34385298
Bug: 34800919
Change-Id: I4d1ad4e2435cc62d38cd7143bc71313e319e952d
2 files changed