Improve the accessibility of the do not disturb switch in the message tray

This branch contains a couple of fixes that improve the accessibility of the do not disturb switch in the message tray for Orca users. Using GNOME shell 3.36 for example, when the do not disturb toggle switch gets keyboard focus, the user hears "push button". When the user presses the enter key or space bar, visually the switch is toggled, but the user hears nothing from Orca as to whether the switch has changed state. This branch associates the label with the toggle switch, and allows the user to be informed of the current state of the switch.