saving the Animation and calling its own stop

instead of calling Animation.stop_all
Bug Fix: stopping roulette causes weird behavior

The Issue: when the stopping touch triggers on_touch_up, there is
one more ``RouletteScrollEffect.trigger_velocity_update``
scheduled after it. Hence the animation cancelling had no effect
inside of ``RouletteScrollEffect.start``.

The Fix: ``RouletteScrollEffect._update_velocity`` now checks for
``is_manual`` before proceeding to the rest of the method.