Simplify snackbar accessible label support

[aomarks]

#412 added a lit directive implementation of @material/mdc-snackbar/util.ts#announce (see the PR for more details).

This directive is mostly a verbatim copy of the MDC utility. It may be possible to do something simpler, more generic, and ideally more suited for server-side-rendering in the future -- but we don't yet have facilities for doing accessibility testing (neither manual or automated), so it is difficult to change the approach without risking breaking something for screen readers.