Refactoring BackGestureTutorialScreen

Extracting logic from BackGestureTutorialScreen (and part of GestureTutorialScreen) into BackGestureScreenViewModel which can now be easily tested, yay!

GestureTutorialScreen.kt now has two similar Composables: GestureTutorialScreenNew (refactored) and GestureTutorialScreen. The plan is to move all gestures to using GestureTutorialScreenNew
and then delete GestureTutorialScreen which is doing too much.
BackGestureScreenViewModel is injected into activity straight away even before screen is needed but it doesn't do anything on its own and lifecycle of its flows is still managed by Composable lifecycle.

Next step: doing the same refactoring for other gesture screens.

Bug: 384509663
Test: BackGestureScreenViewModelTest
Flag: com.android.systemui.shared.new_touchpad_gestures_tutorial
Change-Id: I3e86f103dce5d8853687bf00d4c6546290931ee5