When you say "apk", are you referring to Android packages developed in C++? If so, I'd recommend another forum. I don't think you would get help for that here.
If you mean something else, like apk being shortcut for application, then a Windows executable file will show as icon the icon in the resources that has the lowest ID. Meaning: Just add your icon and make sure it is the one with the lowest ID.
I already told you: Add the ICO as a resource. If you have only one ICO then that's it. If you have more ICO's, then the one that will be shown by Windows Explorer will be the ICO with the lowest resource ID.
i am guessing that to create a custom .ico file is sort of a 'test' of ones programming ability.
to create an .ico file, one either has to use a development or software package that costs money, or write their own program to write .ico files by using a hex editor to analyse preexisting .ico files (unless the header is described somewhere online).
microsoft, in their seemingly limitless sadism have never elicited any method of creating .ico files for the impecunious to my awareness.
to be a part of the 'club' of people who can create an app with a custom icon and thus dazzle users with your ability to be normal seems to require loads and loads of money.
microsoft are not synergists. they are not interested in a world where all people have equal access to expression. if they were, they would have shot charles petzold out of a cannon preferably directly towards the sun or a black hole instead of allowing him to "write" documentation.