Make patchoat match offset when given a patched image

Previously if we gave patchoat a patched image and a already patched
(but improperly relocated) oat file it would not correctly patch the
oat file to the same offset as the image.

Bug: 22599792
Bug: 23119724
Change-Id: I8773022bd75c2e0b7eb529893b147cbd8792bcad