fix(material/table): switch back to class-based styling for flex tables

[crisbeto]

This is a resubmit of #22350 which was merged in by accident.

In an earlier PR we switched the flex-based table styles to target the tag names, rather than classes in order to support flex tables in MDC. The problem with targeting tag names is that it doesn't allow our styles to be applied to other elements within the table. E.g. in #22349 the user isn't able to target the "no data" row.

These changes switch back to targeting classes and add some styles to prevent the flex-specific styles from bleeding into the table-based ones.

Fixes #22349.