Change userid from a uint32_t to a string guid

mash/screenlock/screenlock.h

Index: mash/screenlock/screenlock.h
diff --git a/mash/screenlock/screenlock.h b/mash/screenlock/screenlock.h
index e87995f805b79e6386869e5ad5c38874c4a22557..e6c06119176957e2802fc14435748005dc036e1b 100644
--- a/mash/screenlock/screenlock.h
+++ b/mash/screenlock/screenlock.h
@@ -30,7 +30,7 @@ class Screenlock : public mojo::ShellClient,
  private:
   // mojo::ShellClient:
   void Initialize(mojo::Connector* connector, const std::string& url,
-                  uint32_t id, uint32_t user_id) override;
+                  const std::string& user_id, uint32_t id) override;
 
   // mash::shell::mojom::ScreenlockStateListener:
   void ScreenlockStateChanged(bool locked) override;