fix(a11y): make seek-to-live better announce itself to screen reader users (#5651)

To make the seek-to-live component more accessible, we hide the text that says "LIVE" from screen readers and make it be purely for sighted users. Then set the control text to be either "Seek to live, currently playing live" or "Seek to live, currently behind live" to indicate what this control does and be informative around whether we are at edge or not. Then use aria-disabled on control if we are at edge.

Author: brandonocasey