Draft: implement XAPK as a fully supported Android app format

XAPK is a file format to bundle either Split APKs or an APK+OBB into a single package. It is widely used, but does not so clearly documented.

I haven't found an XAPK file with an OBB to test with, so I haven't implemented that yet. But since this just includes the whole contents of manifest.json, there is probably already enough info in the index-v2 to handle OBB files on the client. Here is more info: https://openxapkfile.net/

@grote IIRC you said you could take on implementing XAPK in fdroidclient once this is implemented. I'll send you the URL for a test repo privately.