[5.4] Fixes phpdocs on abstract ServiceProvider class (Re-fix) (#19432)

Restore phpDocs original parameter class. 

This is a rollback of another Pull-Request (#18248) that breaks other Projects not using the Illuminate\Foundation.

Author: ibpavlov

src/Illuminate/Support/ServiceProvider.php

@@ -9,7 +9,7 @@ abstract class ServiceProvider
     /**
      * The application instance.
      *
-     * @var \Illuminate\Foundation\Application
+     * @var \Illuminate\Contracts\Foundation\Application
      */
     protected $app;
 
@@ -37,7 +37,7 @@ abstract class ServiceProvider
     /**
      * Create a new service provider instance.
      *
-     * @param  \Illuminate\Foundation\Application  $app
+     * @param  \Illuminate\Contracts\Foundation\Application $app
      * @return void
      */
     public function __construct($app)