use oc_preferences instead of oc_encryption to store encyption settings

apps/files_encryption/appinfo/database.xml

apps/files_encryption/appinfo/update.php

apps/files_encryption/appinfo/version

apps/files_encryption/hooks/hooks.php

apps/files_encryption/lib/crypt.php

apps/files_encryption/lib/util.php

apps/files_encryption/templates/settings-personal.php

apps/files_encryption/tests/hooks.php

apps/files_encryption/tests/util.php