Handling back gesture in BackGestureTutorialScreen

The only thing that's reacting to the gesture now is title text changing to "Done"

- registering MotionEvents handler to pass to ViewModel
- GestureTutorialViewModel filters events coming from touchpad
- BackGestureMonitor receives touchpad events and contains logic for recognizing the gesture

Also introducing FakeMotionEvent methods that will be helpful for testing MotionEvents

Bug: 346579074
Flag: com.android.systemui.shared.new_touchpad_gestures_tutorial
Test: manually, see video
Test: BackGestureMonitorTest
Test: GestureTutorialViewModelTest
Change-Id: I516c6d661e5d332fcccc78c21580095ddf90f4a7