Request for `nix-on-droid` in global flake registry

As the potential of `proot` of Android is getting unleashed, [nix-community/nix-on-droid](https://github.com/nix-community/nix-on-droid) is getting more relevant in the developer community. Please make it available by default in the [global flake registry](https://raw.githubusercontent.com/NixOS/flake-registry/master/flake-registry.json)

