[IOS][AIM] Add a leading fade view

Currently the fade view is only applied in the trailing edge.
This CL updates the view to add a leading fade view when the user is
scrolling the carousel.

Change-Id: I8872255be10c5e8c5351cdbd768c64e411ce8a8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965096
Reviewed-by: Christian Xu <christianxu@chromium.org>
Auto-Submit: Ameur Hosni <ameurhosni@google.com>
Commit-Queue: Christian Xu <christianxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1517978}