Basic version of back gesture tutorial view

This is just the "view" part of the gesture tutorial:
- it has placeholder images that will be replaced with animations
- it doesn't actually work apart from done button and back action
- colors and dimensions are not finalized

Bug: 346579074
Flag: com.android.systemui.new_touchpad_gestures_tutorial
Test: manually, see screenshot
Change-Id: Ifc3255d72a4ac652efbbbd80b880f48105dba26f