Accordion behaviour in staggered animation (> 7.0.0)

[OSHistory]

What is the expected behavior?

Prior to Version 7.0.0 a staggered animation will be blend in a 'final'
expansion panel.

What is the current behavior?

The whole panel is faded in unstyled (including contents), only after all animations of the stagger()
function are complete the expansion panels will be displayed correctly.

What are the steps to reproduce?

[Stackblitz Material7]
(https://stackblitz.com/github/OSHistory/test-accordion-stagger)

[Stackblitz Material6]
(https://stackblitz.com/github/OSHistory/test-accordion-stagger/tree/material6)

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

Material 7.0.0

Is there anything else we should know?

[crisbeto]

Should be resolved by #14127.