Allow ElementScenePicker to return `null` (1/2)

This CL changes ElementScenePicker.sceneDuringTransition() so that it
can return `null` in case an element should be drawn/composed in neither
fromScene or toScene of a transition. This will be necessary for
supporting interruptions + movable elements, as we need to know at
composition time which transition is the current transition for a
movable element.

Bug: 290930950
Test: MovableElementScenePickerTest
Flag: N/A
Change-Id: I61c5fd75d1cfdbeb7fbffc923b4b68c9ecfaeef6