fix(material-experimental/mdc-list): properly render leading and trailing icons on list items

[mmalerba]

blocked on #19168

Note: I made some changes to the original list demo in this PR:

1. mat-list-icon and mat-list-avatar were pretty much 2 ways to do the same thing in the old version. In the new version, I'm positioning mat-list-avatar as the bigger leading icon style, and mat-list-icon as the smaller leading icon style (MDC supports both, and they call the larger one an "avatar", so this actually aligns well). I've changed the demo for the old version to show using mat-list-icon in a similar way.
2. One of the nav list demos was showing a not accessible pattern wit a trailing action button. I updated it to be a standard mat list instead of a nav list.

[mmalerba]

This is rebased and ready for review

[jelbourn]

LGTM

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