Fix tokenizer placeholder issue

Summary:
The placeholder property remains null despite it is set to a custom
value.

Test Plan:
Use a custom placeholder in `AphrontFormTokenizerControl`

```
id(new AphrontFormTokenizerControl())
  ->setPlaceholder('My custom placeholder...')
  ->setDatasource('/typeahead/common/projects/');
```

The placeholder should be set to "My custom placeholder..."

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4297