have patchoat take directories instead of filenames

patchoat was already ignoring the basename part of the filename passed
to it for both --output-image-file and --output-image-relocation-file.
We can therefore just pass in the directories instead.

Bug: 72828459
Test: make test-art-host-gtest-patchoat_test
Change-Id: I50f2e19fec1fd27433d3e6ec82df0cfb04c43f21