MatFormField hint and error alignment wrong when using a matPrefix icon

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

#### What is the expected behavior?
Hint and error text are horizontally aligned with input text and both are indented if a matPrefix icon is present.

#### What is the current behavior?
The hint and error text are never indented when matPrefix is used.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-h1lpum?embed=1&file=app/form-field-appearance-example.html

#### What is the use-case or motivation for changing an existing behavior?
Material Design guidelines are not being followed.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 6.0.5, Angular Material 6.2.1

#### Is there anything else we should know?
The alignment spec is not explicitly mentioned, but the anatomy diagram is clear: https://material.io/design/components/text-fields.html#anatomy.

リンクされた StackBlitz の再現から始め、matPrefix アイコンが存在する場合の mat-form-field の動作を調査します。完了条件は、ヒントテキストとエラーテキストが入力テキストと水平方向に揃い、prefix と一貫したインデントになることです。その際、既存のフォームフィールドの動作は維持します。

索引モデルが issue の本文から書いたものです。