MatExpansionPanel animates header height when first rendered

[chemass]

What is the expected behavior?

Given an expansion panel with a header <mat-expansion-panel-header collapsedHeight="10rem" expandedHeight="10rem"> it should be initially rendered at the specified height.

What is the current behavior?

The header is rendered at it's standard height, then animates to the specified height

What are the steps to reproduce?

StackBlitz

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

Angular 7x, Material 7x

[chemass]

Obviously the size I've provided is silly (we actually want a fixed 2rem header), but it shows the issue!
Problem is that we have several vertically stacked panels, so on every render it creates a very noticable shrinking animation.

[Yohandah]

this is a duplicate with #15944

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