fix(material/expansion): not clearing custom query list

[crisbeto]

MatAccordion has a custom QueryList for keeping track of its own headers which we weren't destroying. These changes add a call to destroy it since it has the potential of leaking memory.

[wagnermaciel]

LGTM

[wagnermaciel]

@crisbeto @jelbourn This looks LGTM, but doesn't QueryList already get called when the component is destroyed?
https://github.com/angular/angular/blob/master/packages/core/src/view/view.ts#L547

The docs for QueryList.destroy() just say "internal", which seems to imply that we aren't meant to call this.
https://angular.io/api/core/QueryList#destroy

I'd like to understand why we need to call QueryList.destroy() first, since it should be handled internally.

[crisbeto]

destroy won't be called for this QueryList in particular, because we construct it manually. Only the ones constructed by the framework are cleaned up automatically.

[wagnermaciel]

LGTM again

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