Directly load catalogs from Android assets

This avoids having to copy catalogs from assets into the file system cache first. That's inefficient, and worse, we are actually never updating those copies (and that is non-trivial as assets have no file metadata to notice when that might be necessary).

The downside of this is the need for fmemopen(), which only exists starting at API level 23. We therefore can't use that directly and have to check for it at runtime using dlsym() as well as keeping the old path with all its shortcomings around as a fallback.