Deprecated usage of `OC_App`

[susnux]

This app still uses OC_App which is scheduled for removal.

This usage:

app_api/lib/Controller/ExAppsPageController.php

Line 466 in 3df483f

$appId = OC_App::cleanAppId($appId);

Should be migrated to:

- $appId = OC_App::cleanAppId($appId); 
+ $appId = $this->appManager->cleanAppId($appId); 

cc @oleksandr-nc