assets: Reorganize assets and relocate mupen cfg file to user dir.

This way mupen64plus.cfg is not deleted when the assets are reloaded.
This better mirrors the upstream behavior on other platforms:

 ConfigGetSharedDataPath
    Device-wide data files, typically read-only (e.g. cheat/hack lists)
    Files that are considered integral to (owned by) the app itself
    Linux: /usr/local/share/mupen64plus/
    Windows: C:\Program Files (x86)\Mupen64Plus\
    Android: Context.getExternalFilesDir(null)

 ConfigGetUserDataPath
    User-specific data files (e.g. unzipped hi-res texture files)
    Files that are considered owned by a particular user
    Linux: ~/.local/share/mupen64plus/
    Windows: C:\Users\johndoe\AppData\Roaming\Mupen64Plus\
    Android: Environment.getExternalStorageDirectory() + /mupen64plus/

 ConfigGetUserConfigPath
    User-specific configuration files (mupen64plus.cfg)
    Files that are considered owned by a particular user
    Linux: ~/.config/mupen64plus/
    Windows: C:\Users\johndoe\AppData\Roaming\Mupen64Plus\
    Android: Environment.getExternalStorageDirectory() + /mupen64plus/

 ConfigGetUserCachePath
    User-specific temporary files (currently unused)
    Files that are considered owned by a particular user
    Linux: ~/.cache/mupen64plus/
    Windows: C:\Users\johndoe\AppData\Roaming\Mupen64Plus\
    Android: Environment.getExternalStorageDirectory() + /mupen64plus/