Improve notifications screen animation

Actual behavior

• On the notifications tab, when you swipe to move between notifications, the current notification (A) is shown a level above the next notification (B) on the Z-axis.
• The next notification (B) is slightly reduced in size and opacity, but then animates to full size and opacity from behind the other one (A)
• The initial notification (A) moves horizontally out of the device frame.

Here's a sketch and a very rough video to describe it.

https://user-images.githubusercontent.com/50576199/131374146-93a0a1c2-36a3-4574-a500-f074bd584b3f.mp4

Issues

• In the current solution, because notification B is 'behind' and smaller than notification A, I think it implies that there is a hierarchical or child/parent relationship between these two.
• It feels like the app is going up/back to the notification screen.

Expected behavior

• These notifications are actually at an equal level of hierarchy. They are in a list. But they don't have a parent/child relationship. They are siblings of equal level.
• It might work better if they all just moved horizontally, with no shrinking or transparency.
• We could avoid the overlay issue entirely. The notifications would just swiped left/right on an infinite horizontal plane. It would be like they are all on one long sheet, and the device screen is a window framing one of them at at a time.

Here's a sketch and a very rough video to describe it.

https://user-images.githubusercontent.com/50576199/131374650-2794b1b3-fccd-415a-b9e6-ae6c09fbe64f.mp4

PS: Description is taken from https://github.com/wordpress-mobile/WordPress-Android/issues/14919#issuecomment-908503715.

Start at the notifications tab implementation and compare the current swipe transition with the attached sketches and videos. Clarify the intended horizontal transition and its acceptance criteria before changing it, then verify that swiping between notifications treats them as equal-level items without the current hierarchy cues.