Fix ArtDexFileLoader::GetMultiDexChecksums to handle .dex files properly

Runtime::RegisterAppInfo relies on
ArtDexFileLoader::GetMultiDexChecksums to check if the input files have
dex code, but that method doesn't propertly return dex_locations if the
input is a .dex file. This issue caused profiles not to be written for
.dex files. This CL fixes it.

Bug: 282191456
Test: art/test.py -b --host -r -t 595-profile-saving
Test: m test-art-host-gtest-art_libdexfile_tests
Change-Id: Ia8597fa9c7e7c7c9a82df0708fe0867216362423