Memory leak with media query listeners

[JoostK]

Bug Report

What is the expected behavior?

Registered event handlers are unregistered on destroy to be able to free memory.

What is the current behavior?

This media query listener is never unregistered, leasing to memory leaks:

flex-layout/src/lib/core/match-media/match-media.ts

Line 115 in 36f8f39

mql.addListener(onMQLEvent);

What are the steps to reproduce?

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

In unit tests it's typical to setup and teardown an application, in which case memory from a previous run should not be retained.

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

Observed in flex-layout@9.0.0-beta.29 with Angular 9.0.4.

Is there anything else we should know?

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