Add fragment transition animations to Wallpaper Picker

While this CL does improve the transitions a lot (in my view), there are still a number of issues with the transition animations which I will address in subsequent CLs:
1. The SurfaceViews: For now, the SurfaceViews are set to GONE in the exiting fragment when the transition animation starts. That's done because the transitions do not work well when there are SurfaceViews both in the disappearing and the appearing fragment. I'm still investigating whether it is possible to take a screenshot of the SurfaceView and display that during the transition instead of the SurfaceView itself. Otherwise displaying a placeholder rectangle might be our best option.
2. Shortcuts transition: The asynchronous StateFlow based implementation of the shortcuts feature updates some view state after the transition has started, causing e.g. the horizontal shortcut picker to scroll after the transition has ended. If possible, we should prevent that by postponing the transition.
3. Activity transitions: Clicking the preview on the StartScreen starts an activity transition which looks very different from all other transitions. There are multiple other activity transitions in the whole app as well. They should be changed to match the fragment transitions.
4. Shared elements: I see some potential improvements to the transitions by adding shared elements. (E.g. the top navigation bar). I will reach out to the design team to see what they think.

Bug: 278929581
Flag: WALLPAPER_PICKER_PAGE_TRANSITIONS
Test: Manual, i.e. analysing screen recordings of fragment transition animations
Change-Id: Ie1151edc0f53c16e5597a24715f8b9dd431d1f83