MatStepper treats steps as `interacted` if you immediately return to a previous step

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

Bug

#### What is the expected behavior?

Angular-material forms should be presented in a non-error state when the conditions of the default ErrorStateMatcher are not met (not touched or not submitted)

#### What is the current behavior?

When a form is presented inside a mat-stepper, when traversing to a previous step, the form will be in an error state even if the form is pristine and un-submitted. Any attempt to try to reset the form or FormGroupDirective fails.

#### What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-fay224

#### What is the use-case or motivation for changing an existing behavior?

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

#### Is there anything else we should know?

从链接的 StackBlitz 复现开始，跟踪返回上一步时 MatStepper 与表单及默认 ErrorStateMatcher 的交互。完成标准是：未修改且未提交的表单在重新访问某一步时仍保持非错误状态，并且重置表单或 FormGroupDirective 能按预期工作。

由索引模型根据 Issue 内容生成。