[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

Start at the Angular Material FormField implementation and trace how hint/helper text is currently rendered. Confirm the option's expected focused and unfocused states, then verify that the hint is hidden without focus and remains visible when focused.

Written by the indexing model from the issue text.