[Samsung Galaxy A8 (2018) Questions & Answers] Ascii3:
For decompiling and building Android Oreo, I prefer version 2.3.1 of APKTool to other versions of APKTools.

I have not tried modifying framework-res.apk of a Samsung Android Oreo nor have I particularly tried the location services mod you are attempting (though I might get around to trying it someday) and do not know if it is valid to to accomplish what you want with it, but believe that the process should be similar to modding the file on LG Android Oreo. I shall try to guide you to how to prepare a framework-res.apk that is proper.

To answer your question about expecting a certain key. The answer to that is that that is usually the case. The signing scheme checks on system apps; however is usually not as thorough as non-system apps. framework-res.apk is also special in that it is not a running app and is instead used as a cache of system resources and system meta information. In the past, before Android Oreo, a rebuilt framework-res.apk may be made to work simply by including original signature files (META-INF) and corresponding AndroidManfest.xml file from the original framework-res.apk into the rebuilt fraemwork-res.apk file. The system would evaluate these files, and pass a check for valid platform signature. With Android Oreo, it appears that there is an additional check that was not present in the past; my best guess is that the system is checking for the V2 signing scheme signing block within the V2 Signing Scheme APK file structure. The check does not, however, thoroughly validate the signing block information.

Your mod seems rather simple and, given your previous posts, would only involve a modification to framework-res.apk "resources.arsc" member file (which contains the compiled "res/values/arrays.xml" file). If the rebuilt "resources.arsc" can be used to update ("Update" is an actual ZIP archive operation) the original framework-res.apk's member file, the updated framework-res.apk should work (and remained zip-aligned if originally zip-aligned), so long as the packed file size of the updated member file is less than or equal to the packed size of the original member file, plus up to 4 bytes depending on proximity to the next member file data if original framework-res.apk is zip-aligned as expected. If the packed modified member file(s) are larger, the original APK file structure would likely not be preserved, and a different method might have to be used. Also note that 7-Zip is not reasonable software to use for this, despite it being included with many tools on XDA to modify APK files; the software has had a history of rearranging unnecessarily zip file table entries when a change is made to an archive. Use a different tool that does not do this, such as WinRAR (I have tested version 5.61).

For normal apps, one may not copy over "resources.arsc" or resources from two different app builds and have things work correctly when the app runs; one would also need to make corresponding changes in the *.dex APK member files. framework-res.apk, not being an app the runs, has no corresponding *.dex files, and one need not worry about corrupting the relationship between the *.dex files and the resource files because none exists to corrupt.