Add handling for touch outside billing window.

DebugBillingActivity.kt

override fun onCreate(savedInstanceState: Bundle?) {
            // ...
    window.addFlags(WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL)
    window.addFlags(WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH)
  }

  override fun onBackPressed() {
    broadcastUserCanceled()
    super.onBackPressed()
  }

  override fun onTouchEvent(event: MotionEvent?): Boolean {
    if (event?.action == MotionEvent.ACTION_OUTSIDE) {
      broadcastUserCanceled()
    }
    return super.onTouchEvent(event)
  }

  private fun broadcastUserCanceled() {
    broadcastResult(BillingResponse.USER_CANCELED, Bundle())
  }