Use app ID instead of object

Fixes several error messages when installing an app from the appstore, including:

```
Illegal offset type in isset or empty at /media/psf/stable9/lib/private/legacy/app.php#662
Illegal offset type at /media/psf/stable9/lib/private/legacy/app.php#663
Illegal offset type at /media/psf/stable9/lib/private/legacy/app.php#661
Object of class OC_App could not be converted to string at /media/psf/stable9/lib/private/legacy/app.php#81
trim() expects parameter 1 to be string, object given at /media/psf/stable9/lib/private/legacy/app.php#628
```

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>