Pass class="nixOnDroid"; to evalModules

This is useful for getting better error messages when mixing modules; a module designed for nixos configs sets `_class = "nixos";` and then when you try to use it from a nix-on-droid config it shows an error.

See: https://nixos.org/manual/nixpkgs/unstable/#module-system-lib-evalModules-param-class

modules/default.nix

@@ -25,6 +25,7 @@ let
   rawModule = evalModules {
     modules = [ configModule ] ++ nodModules;
     specialArgs = extraSpecialArgs;
+    class = "nixOnDroid";
   };
 
   failedAssertions = map (x: x.message) (filter (x: !x.assertion) rawModule.config.assertions);