Make System tray a bit more accessible

Now when opening the popup containing the hidden elements, they can be keyboard navigated and expose their interaction to the screen reader.

Test Plan

1. Add keyboard shortcut to the system tray applet
2. Trigger it
3. Navigate in the system tray popup by only using the keyboard.

Not covered by this MR

The accessibility of the individual applets and making the more important applets also selectable from the popup.

CCBUG: 352476

@chempfling