Rewrite of OnOffSwitch input handling for desired behavior and simpler internal state (only one listener, not two). See https://github.com/phetsims/acid-base-solutions/issues/107.

More specifically:
  "Click" with drag less than threshold will toggle (on thumb or track).
  "Drag" with drag more than threshold will change slider value to the side that it is on (drag starts on thumb or track).
  "Touch Snag" will behave the same as above, but will only activate on the thumb (not the track).
  The drag threshold is customizable in the options passed in.