[A11y] aria-describer is not reactive

[phyrephox]

What is the expected behavior?

When the aria-label changes it should determine whether to continue describing use aria-describedby or let aria-label do it.

What is the current behavior?

It only checks on construction time whether something should be labeled.

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

https://stackblitz.com/edit/angular-k4aktm

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

Angular material 8.0.0

Is there anything else we should know?

Related to #15048 and it's fix. However, when the aria-label and/or tooltip text come from the component typescript rather than hardcoded in the template, both an aria-label and an aria-describedby are set in the final html, which means screenreaders will read this twice.