Replace remaining shell references with service manager

mash/init/manifest.json

 {
   "manifest_version": 1,
   "name": "service:mash_init",
   "display_name": "Root Controller",
   "capabilities": {
     "provided": {
       "mash_init:init": [ "mash::init::mojom::Init" ]
     },
     "required": {
       "*": [],
       "service:login": [ "login:login" ],
-      "service:shell": [ "shell:user_id", "shell:all_users" ]
+      "service:service_manager": [
+        "service_manager:user_id",
+        "service_manager:all_users"
+      ]
     }
   }
 }