Add pressed state to on/off button

A darker surface color for the main on/off button was defined in Figma for the pressed state, which is still missing in the UI implementation.

See the attached screenshot, or directly at https://www.figma.com/file/sjNWeIOpb0BckjmxApXd5m/Tor-VPN?node-id=950-3684&t=3JAO3fLARdhHoQR1-0