Dense lists avatar size doesn't follow the guidelines

[philip-firstorder]

Bug, feature request, or proposal:

Dense lists avatar size doesn't follow the guidelines, it should be 36px in height and width.

What is the expected behavior?

Dense list avatar size should be 36px;

What is the current behavior?

In material2/src/lib/list/list.scss
$mat-list-avatar-size = 40; For both normal and dense lists.

What are the steps to reproduce?

Dense list to the right should have smaller avatars.
https://tina-material-tree.firebaseapp.com/list

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

Avatar size should follow the guidelines.

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

"@angular/core": "^5.2.5",
"@angular/material": "^5.2.1",

Is there anything else we should know?

[philip-firstorder]

The height should remain 48px when using mat-nav-list with mat-list-icon.
https://material.io/design/components/navigation-drawer.html#specs

@crisbeto Now because of the mat-list-item-with-avatar it gets set to 56px, which is wrong.
https://angular-gqqput.stackblitz.io

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