runtime: Correctly handle data dirs when in a Flatpak environment

• Expose the modified /app to the subsandbox, if any

  If we modify "/app" we want to bind it to the new subsandbox instead of always binding an empty directory.

• When we are in a Flatpak environment there are two issues:

  • It's common to have multiple data directory, usually one for every multiarch
  • If we bind the data directory in "/usr/share/${dir_basename}" there is no guarantee that it will be in the library search path

  For this reason we avoid raising a warning if we have multiple data directory and also we bind them preserving their original path.