im using fopen (C++) to load files directly from sdcard/internal memory <- i would like not to change that but loading things through assets and then saving them to sd card seem pretty lame since game will take 2x more space than it should.
I dont want to change the loading code.
The ideal thing would be that apk (game only could be downloaded with all game files - without any additional download from my app.)
So during apk installation it could extract files to desired path, then delete the zipped content.
I'm pretty sure you can't delete the apk contents, I'm afraid.
One option would be for you to disable compression of your files within the apk. Certain extensions like .mp3 .zip, etc don't get compressed, so if you rename your game files to one of those then you can disable compression, alternatively you can get your hands dirty and figure out whereabouts in the build process the compression decision is made and turn it off completely or just for your particular file extensions.
With an uncompressed asset in your apk, you can use AAsset_openFileDescriptor to get a file descriptor for a package file, then you can use fdopen followed by fread/fclose. So you'd only have to rewrite your file opening code and not your file manipulation code which might be a less daunting prospect.
Obviously if the build process isn't compressing for you, then you probably ought to be compressing your game files yourself, plus it's worth noting that the docs don't really make promises that AAsset_openFileDescriptor will work. I'm pretty certain that AAsset_openFileDescriptor does always work for non-compressed main apk assets and that it's unlikely to ever change on future revisions, but that's very different from it actually being a guarantee in the documentation.
All that said, you're probably best introducing a layer of abstraction in your file loading code and just using AssetManager functions on Android.