Add a stub QuotaPermissionContext implementation

This prevents the crash when the application requests
persistent storage access using the Quota API (e.g. on
http://codepen.io/matt-west/full/CrfKh). The request has been
disallowed for now, because it should rely on user permission.

Change-Id: I4ae057c9485d3f06f45a637c7eeda9dd69fe6b54
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>