Animating transition from looping starting animation to in-progress animation

Before one animation would be switched with another one (with flicker in between) and now we're fading in new animation
on top of old animation. Old animation below just disappears when new animation is fully fade in.

Bug: 346579074
Test: Open back gesture screen, start doing gesture and see smoother animation transition
Flag: com.android.systemui.shared.new_touchpad_gestures_tutorial
Change-Id: I62bb8fc4e605f04a998c41e4f549c03ba3104b1f