fix(material/list): use flexbox to size mat-line elements (#19809)

[swseverance]

• use flex: auto instead of width: 100% to allow .mat-list-text element to take up the available space

stackblitz demonstrating issue

Before

After

[jelbourn]

Seems okay at first glance, but we'll have to see if this causes any visual diffs inside Google (this process can be pretty slow)

Can you expand on the commit message? Saying "fix layout bug" doesn't really provide enough information about what this change does and why. The format would be something like

fix(material/list): use flexbox to size mat-line elements

<explanation here about why `flex: auto` is better than `width: 100%`>

[swseverance]

@jelbourn thanks. I updated the commit message.

[jelbourn]

LGTM

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