[FormField] Add option to allow hint to be shown only when focused

#### Bug, feature request, or proposal:
Feature request

#### What is the expected behavior?
When the option is specified, the hint/helper text is hidden unless the field has focus.

#### What is the current behavior?
The hint is always displayed unless no hint is used

#### What is the use-case or motivation for changing an existing behavior?
Material Design specifications call for the hint to be either always displayed or displayed on focus only: https://material.io/guidelines/components/text-fields.html#text-fields-layout (see 'Helper text').

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Material v5

从 Angular Material FormField 实现入手，跟踪当前提示/辅助文本的渲染方式。确认该选项预期的聚焦和未聚焦状态，然后验证提示在未获得焦点时被隐藏，并在获得焦点时保持可见。

由索引模型根据 Issue 内容生成。