Bug 1866100 - improve PromptUtils activeness handling, a=RyanVM

  this.focusTarget.addEventListener("keydown", this, true);

  this.focusTarget.document.addEventListener("unload", this);

  // If we're not part of the active window, don't even start the timer yet.

  let topWin = focusTarget.browsingContext.top.window;

  if (topWin != Services.focus.activeWindow) {

    return;

  }

  this.startOnFocusDelay();

};