fix(material/snack-bar): prevent override of snack bar action button color

[json-derulo]

Prevents the color (if specified) of the snack bar action button from being overridden to the accent color, when using the matSnackBarAction directive.

Closes #27328

[mmalerba]

@crisbeto can you confirm if this was intended to only apply to unthemed buttons

[vmuresanu]

Just stumbled upon this issue. @mmalerba @crisbeto any chance this PR is merged so that user can override the --mat-mdc-snack-bar-button-color variable?

[e-karlsson]

Any chance this will be merged soon? @crisbeto Or if there is any other way to get it fixed to override the color of action button in snackbar

[json-derulo]

@mmalerba @crisbeto I fixed the merge conflicts, the changes merge cleanly again. Is there something else blocking this PR from being merged?

[mmalerba]

I think just the Google internal tests. I kicked off a run, lets see how it goes