Add scroll buttons to Calendar up next view

Main changes:
- Add scroll buttons for calendar up next view
- Show single event as full width with no scroll buttons
- Add scrolling animation when scroll buttons are pressed

Smaller changes / refactors:
- Fix "Up next" view not displaying on initial load
- Add translation screenshots
- Add docs
- Remove static FakeNow methods from tests

Screenshots and recordings:
- Single event scroll buttons hidden http://shortn/_H4w3y6uXf8
- Multi events scroll buttons visible http://shortn/_glwCpdUgP3
- Scroll buttons behaviour http://shortn/_Ru7YX4RD9H

Bug: b:258649456
Change-Id: I1ad2b6f304eaf977857f4ade4f5aed900c930099
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4051265
Reviewed-by: Jiaming Cheng <jiamingc@chromium.org>
Commit-Queue: Sam Cackett <samcackett@google.com>
Cr-Commit-Position: refs/heads/main@{#1079754}