We should use UserID object to identify users instead of username.

components/proximity_auth/ble/proximity_auth_ble_system.h

Index: components/proximity_auth/ble/proximity_auth_ble_system.h
diff --git a/components/proximity_auth/ble/proximity_auth_ble_system.h b/components/proximity_auth/ble/proximity_auth_ble_system.h
index aa0ac0a8e84b723913c343af5d20ea9db8bda726..75f062efeb866c7a92b63670c9b5dc26252eb417 100644
--- a/components/proximity_auth/ble/proximity_auth_ble_system.h
+++ b/components/proximity_auth/ble/proximity_auth_ble_system.h
@@ -17,6 +17,10 @@ namespace device {
 class BluetoothGattConnection;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace proximity_auth {
 
 class BluetoothLowEnergyConnection;
@@ -38,7 +42,7 @@ class ProximityAuthBleSystem : public ScreenlockBridge::Observer {
       ScreenlockBridge::LockHandler::ScreenType screen_type) override;
   void OnScreenDidUnlock(
       ScreenlockBridge::LockHandler::ScreenType screen_type) override;
-  void OnFocusedUserChanged(const std::string& user_id) override;
+  void OnFocusedUserChanged(const user_manager::UserID& user_id) override;
 
  private:
   // Handler for a new connection found event.