Rename mw-ui-text class to mw-ui-quiet

No other changes, no visible changes.
This is for compatibility with the updated Agora mw-ui styles  that will
some day be in core (I1dd9bfac).

Bug: 56641
Change-Id: I0f46d681f4ff81a4f83e9935b601c544dcd4e2dc
Merge "Rename mw-ui-text class to mw-ui-quiet"