video Play button needs aria-label, not title

In ye olden days, we used title attributes for labeling. But then title attributes might be redundant with other labeling (such as aria-label), so a lot of screen reader users turn off reading of title information in their screen reader's verbosity settings. It's kinda broken that way now, and we don't have a good solution except to avoid title attribute unless needed for sighted user usability on graphical controls etc.

In the standard video playback xBlock, the Play button is currently labeled with a title attribute, so some screen readers might not read it.

Desired state: use aria-label instead of title for naming the button.

This may not be a WCAG issue but affects screen reader usability.